Skull Anatomy

Picture from Robswatski (http://www.flickr.com/photos/rswatski/)

The human skull is part of the skeletal system. It’s structures (namely the cranium and facial bones) provide the anchorage for the soft tissues of the face and as a result, their form (both shape and quality) dictate much of the appearance of the face. In adult humans the skull consists of 22 bones. All of these are bound together by sutures (a fibrous joint only found in the skull), the only exception being the mandible. These joints are synathroses (joints that do not allow much movement); immovable joints formed via ossification. The small amount of flexibility present is primarily as a result of Sharpey’s fibres.

Human Skull bones have many features. Extra tissue on the bones (the function being to hold muscles and ligaments) are known as Processes. Grooves in the bones (formed during development) are known as Lines. Small cavities within structures of the skull (primarily for the housing of blood vessels and nerves) are known as Foramina.

Picture from Robswatski (http://www.flickr.com/photos/rswatski/)

Perhaps the most notable part of the human skull is the cranium. This is the area which houses the human brain and brain stem in a large cavity commonly known as the cranial vault. Eight plate-like bones form the brain case (neurocranium). Of these the most profound in terms of its effect on  the appearance of the bearer’s face is the frontal bone. The human skull also contains fourteen facial bones (the splachnochranium). These bones provide the foundations of the human face. The most important facial bones (though it can be said that they’re all very important!) are the maxilla (upper jaw), mandible (lower jaw) and zygomatic (cheek bone).

Picture from Robswatski (http://www.flickr.com/photos/rswatski/)

The human skull forms many cavities such as those housing the eyes, internal ear, nose, mouth and of course the sinus cavities. The function of the sinus cavities is the subject of some scientific debate: they’re lined with respiratory epithelium and as such are considered primarily to warm and moisten air entering through the nasal cavity however it is also important to note that they reduce the overall weight of the skull without significantly effecting the strength.

Below I have divided the human skull into its 2 major components. Within these lists are the majr features of these bones.

Facial Bones

Lacrimal Bone: This is a small bon e located at the inner corner of the eye socket (medial orbit). It is the most fragile bone in the face, has two surfaces and four borders.

Picture from Cuttienut http://www.flickr.com/photos/54066547@N02/

Mandible: This is the lower jaw. It houses the lower teeth and is notable as the only part of the human skull that is mobile (e.g. chewing, speech etc.). It is a fusion of two halves down the mental symphysis.

Picture from Guccibear http://www.flickr.com/photos/guccibear2005/

Maxilla: This is the upper part of the jaw. It houses the upper teeth and is actually a fusion of two bones along the palatal fissure.

Piture from Cutienutt http://www.flickr.com/photos/54066547@N02/

Nasal Bone: Located between the eye sockets. They are actually two bones that together form the nasal bridge. Each has two surfaces and four borders.

Picture from Wessex Archaeology http://www.flickr.com/photos/wessexarchaeology/

Vomer: This is located in the nasal cavity on the centreline of the nose.

Picture from RachelHermasillo http://www.flickr.com/photos/rachelhermosillo/

Zygomatic Bone: This is the primary bone in the cheek. It articulates (provides a joint) with numerous other bones within the skull (maxilla, frontal bone, sphenoid bone, temporal bone). In addition to this it forms part of the lateral wall and floor of the orbit. It has four processes and four borders.

Picture from trp0 http://www.flickr.com/photos/trp0/

Cranial Bones

Ethmoid Bone: This is a bone forming part of the eye socket. It separates the nasal cavity from the brain. It articulates with fifteen other bones (the volmer, the frontal bone and sphenoid, two nasal bones, two maxillae, two lacrimals, two palatines, and two inferia nasal conchae). It has three major components: the cribriform plate, the ethmodial labyrinth, and the perpendicular plate.

Picture from Robswatski http://www.flickr.com/photos/rswatski/

Frontal Bone: This is the bone comprising what is commonly known as the forehead (squama forntalis). It houses the frontal lobes of the brain. It forms the upper regions of the eye sockets (pars orbitalis) and forms part of the nasal cavities.

Picture from Ted Zhu http://www.flickr.com/photos/47771364@N04/

Occipital Bone: This is a major structure in the cranium located at the lower back area. It has a large opening known as the Foramen Magnum (an example of previously mentioned foramina) through which the medulla oblongata enters (or exits) the cranial vault. It also transfers the spinal accessory nerve, vertebral arteries and the anterior and posterior spinal arteries.

Picture from RachelHermosillo http://www.flickr.com/photos/rachelhermosillo/

Parietal bones: These bones together form the sides and top of the human skull. They are located between the frontal and occipital bones and are roughly square (though curved as to form a rounded surface). Each bone has two surface and four borders.

Picture from RachelHermosillo

Sphenoid bone: This forms a section of the eye socket. It is butterfly shaped and has a protective cavity which houses the pituitary gland. 

Picture from Robswatski http://www.flickr.com/photos/rswatski/

Temporal bone: The temporal bones are located in the are referred to as the temple. They extend behind the ear toward the mandible  and house the temporal lobes of the brain.

Picture from RachelHermosillo http://www.flickr.com/photos/rachelhermosillo/

Think you’ve got that down?

Cardiac Muscles 2

The human heart starts developing very early in embryonic life and even before it is completely formed it starts beating by about 22 days after the fertilization has occurred. The heart has to begin working very early as in an early embryo, growth is at a furious rate. Every 4 hours the early embryo doubles in mass and the cells need the nutrients and oxygen.

The hearts of all vertebrates like - fish, frog, lizards, birds, mammals are myogenic, that is, they work without any stimulus coming from the nervous system. External stimuli change the heart rate but do not initiate the heart beats. For some invertebrates like insects this is not the case. Their hearts are neurogenic. That is unless there is a stimulus form the CNS their heart does not carryout its pumping activity.

In humans a mature cardiac muscle cell (fiber) is cylindrical in shape and is about 7 times as long as it is broad. Its length is approximately 100 micrometers (microns) and width about 15 micrometers (microns). The heart muscle cells contain numerous mitochondria and are resistant to fatigue. The heart muscles may not be able to contract as powerfully as skeletal muscles do but they do not easily get tired. Perhaps about 40% of the volume of cytoplasm of these cells is of mitochondria. In case of skeletal muscles the mitochondria would occupy about 2% volume. Auricular cardiac muscle cells are somewhat smaller in size than the ventricular cardiac muscle cells.

Between the right and left auricular cardiac muscle cells the difference is - more auriculin (ANF) granules are found in the right auricle. (for ANF details kindly refer Cardiac Muscles - I, the previous article in this series)
Cardiac muscle cells are of two basic types - the contractile cells and impulse generating non-contractile cells. The impulse generating cells of the heart make it possible for the atria (auricles ) and ventricles to contract in a rhythmic manner.

These impulse generating cells of Sinu-Atrial Node are smaller than the contractile cells. Also they contain less numbers of myofibrils.The SA Node in each species has its own specific rate at which it initiates the contraction of heart and sets the heart beats. For humans it is about 72 times a minute. For an elephant about 35 per minute and for a dog about 90. For very small animals like a mouse 500 times and a humming bird's heart beats over 1000 times a minute. By and large, larger the animal lesser is the heart rate.

In an embryo the heart rate is low and as the embryo grows the heart rate increases. Later as the animal becomes old there is reduction in the number of times the heart beats.

The copyright of the article Cardiac Muscles-II in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish Cardiac Muscles-II in print or online must be granted by the author in writing.

Cardiac Muscles 1

The heart has muscles. The muscles are only in a single layer of heart i.e. in myocardium. From outside inward the layers of heart are - 1. fibrous pericardium, 2. serous pericardium ( 2.1 parietal and 2.2 visceral layer i.e. the 2.2 epicardium), 3. myocardium and 4 .endocardium. The myocardium is the thickest layer of human heart. 

 

The muscle in the myocardium is neither of smooth nor of skeletal type. It is a special type of muscle found only in the heart and is called cardiac muscle. The cardiac muscle is striated and has sarcomeres but is involuntary. In other words you can not make your heart stop or start or go slow or fast just by wishing or ordering it to do so. The cardiac muscle has cylindrical fibers arranged parallel to each other. But the bundles of muscle fibers branch and are joined to each other forming a three dimensional network. Take a close look at cardiac muscle - http://webanatomy.net/anatomy/cardiac_mu... Ends of cardiac muscle fibers are thrown into many folds and form intercalated discs.

These are straight or steplike dark lines at interfaces of cardiac muscle fibers. Cardiac muscle cells show tiny granules. These are the precursors of a hormone. Till recently no one everthought that heart would be involved in making a hormone but now it is clear that heart does produce a hormone. Especially right auricle (and to a smaller extent even left auricle ) produces a hormone called ANF (Auriculo Natriuretic Factor ) also called as auriculin or atriopeptin. This hormone is a peptide i.e. holds a few amino acids together by peptide bonds and it acts on kidneys to promote excretion of sodium and water.
The heart muscle scan relax between periods of contractions. They have long refractory period.

The cardiac muscle contract rhythmically about 75 times a minute. A little slower when a man is sleeping and much faster when a man is running, digging etc. The heart muscles contract on their own and do not need any external prodding in the form of a stimulus. Nobody knows how this spontaneous contraction takes place.
The heart has natural pacemaking activity. The natural pacemakers of heart are three groups of specialized cardiac muscles.

They are the - sinuauricular node, auriculoventricular node and the Bundle of his. The SAN i.e. the sinuauricular node located in wall of right atrium or right auricle is the prime pacemaker. Have a look at the S.A. Node at http://sprojects.mmi.mcgill.ca/cardiophy... The S.A.Node generates the contraction events 75 times a minute. The AVV i.e the auriculoventricular node can also generate the contractions of heart at a lesser rate of about 50 times a minute or so. The Bundle of His can generate the contractions of heart at an ven lesser rate of about 30 times a minute.

The copyright of the article Cardiac Muscles- I in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish Cardiac Muscles- I in print or online must be granted by the author in writing.

Hair on human ear

I hope you have already taken the quiz published in this topic on 21st June, 2004 and are waiting to see how many of your answers are matching with the answer key.

Here are the answers - Answer key -Q 1=d, Q2=b, Q3=a, Q4=a, Q5=b, Q6=c, Q7=d, Q8=b, 9=d, Q10=c, Q11=c, Q12=b, Q13=b, Q14=c, Q15=a .

I look forward to your comments and queries on the questions and answers.
Today I shall like to discuss with you about a point in genetics in biology. It is about hair on human pinna that is the external ear.

There are some people who have a tuft of hair on their ears. You will always find them to be males and their age group will be 35 plus. Take a look at this link please, to see the character trait I am talking about - http://www.people.virginia.edu/~rjh9u/ha...

Several text books of biology at the Higher secondary level (XII) in India mention the condition as hypertrichosis and explain it is located on the Y chromosome.

The gene is reported to be present on Y chromosome ( non-homologous part ) which is present only in males. Since the gene for hypertrichosis is claimed to be present only in males it is called a holandric gene i.e. wholly andric ( male ) gene. Naturally the gene is passed on from father to son on the Y chromosome. Since only male children of a man will inherit the Y chromosome this gene follows straight inheritance that is it goes from a man to his son and then to his grandson always through the male line.

Obviously this is incorrect information in the light of new information available from human genome project. http://www.utexas.edu/courses/gene/L07.h... All modern literature as in link referred in previous line mentions only SRY gene ( Sex Determining Region on Y, earlier known as TDF, the Testicular Determining Factor )
The information about hypertrichosis gene on Y chromosome is still dished out to students by many teachers who believe what ever is in print in a text book must be true.

Parents of students even in medical and biology related fields have never raised any doubts about it.
And letters written to the authors of text books draw a blank.

Perhaps this not a unique case. In other states of India and other parts of the world such erroneous ideas are perhaps still in circulation. I shall be happy to get feedback from subject experts, teachers of biology, students currently studying and those who have passed out but continue to take interest in academics, educators and any one interested and cares about biology education.

Rarely one still get references through links discussing human hairy pinna as Y-linked trait. e.g. http://www.meredith.edu/biology/genetics...

 

The copyright of the article Hair on human ear in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish Hair on human ear in print or online must be granted by the author in writing.

Back muscles

Kindly read answers to Muscle quiz-II, match them with your answers and continue with this article.
In case you have landed up here directly and wish to take the quiz / quizzes you may please click on (1)http://www.suite101.com/article.cfm/our_... and (2) http://www.suite101.com/article.cfm/our_...
Answers for the Muscle muscle quiz -II. Hope you have got most answers correct. Answer key to muscle quiz -II - 1=d2=b3=a4=b5=b 6=b7=b8=a9=a10=c 11=c12=b13=b14=d15=a

The back is a wide flat part on our dorsal side extending from the base of neck to end of spine. The backbone i.e. the vertebral column is not one single long bone. In adults it is a series of 26 bones attached to each other. The total number of vertebrae in children is 33. Each small bone that makes the backbone is called a vertebra. Between two successive vertebrae is a cushioning disc, the intervertebral disc. The back is held upright by muscles, tendons and ligaments attached to the backbone.

The spine with its vertebrae separated by intervertebral discs gives our back strength coupled with flexibility. Muscles at the back effect various types of movements. Thus the back appearing so broad and rigid like a plank is a dynamic structure.

Some of the major muscles at our backside are - (1) The Trapezius is a muscle of shoulder blade but two of these muscles form a diamond like shape and cover upper back. Fibers of the trapezius have an origin from the base of skull and first neck vertebra upto the last i.e. 12 th thoracic vertebra. It is inserted on collar bone, and on shoulder blade. The trapezius moves collar bone and steadies or moves the shoulder blade. (2) Latissimus dorsi - the broad fanlike muscles in middle portion of back. They originate in last 6 thoracic vertebrae and 5 lumbar vertebrae. The lumbar region is between the ribs and the hip bones. These muscles end up on the arm bone and bring arm near the body or can take it away as well as can move shoulders downward and backward. These muscles can also rotate arm. (3) Erector Spinae Muscles - This is a group of muscles or a muscle complex. It is a large mass composed of muscles and tendons. It spans from the neck to small of back that is almost entire length of the back. The erector spinae has three different layers of fibers. (3.1) The iliocostalis or lateral layer, (3.2) The longissimus or middle layer and (3.3) The spinalis or medial layer.

Together all these layers of the erector spinae extend the trunk. (4) Rhomboides major and minor - These two muscles extend from the medial margin of shoulder blade to the backbone. They help in bringing about movement of the scapula i.e. the shoulder blade. Here is a site for taking a close look at the muscles of our back http://www.bartleby.com/107/illus389.html
 

The copyright of the article Back muscles in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish Back muscles in print or online must be granted by the author in writing.

Brain Anatomy

(click pictures to enlarge) FIG. 1

    The brain is the centre of the nervous system and as such is an incredibly complex organ. It is required to receive process and respond to sensory information; control motor function; maintain homeostasis whilst all the time coordinating our perception and thought. Each area of the brain is responsible for separate tasks as part of its overall function, though it is interesting to note that in cases of certain damage, different areas may take on some of the roles of the defunct brain matter.
The Human brain is divided into three main areas:

• The Hind brain (rhombencephalon)
• The Mid-brain (mesencephalon)
• The Forebrain (prosencephalon)

    The forebrain and the hindbrain are also divided down the sagittal plane into hemispheres. These hemispheres have lobes which are derivative of specific functions.

The Hind brain/Rhombencephalon

 

FIG. 2

    The  Hind brain/Rhombencephalon is located within the posterior fossa of the cranial cavity. The Rhombencephalon has the following functions:

• Attention and Sleep
• Autonomic Functions
• Complex Muscle Movement
• Conduction Pathway for Nerve Tracts
• Reflex Movement
• Simple Learning

It is made up of a number of several important structures. These include:

•  The Myelencephalon

1. Medulla Oblongata

FIG. 3

    The Medulla Oblongata is the part of the brain following on immediately from the spinal cord. The  medulla is responsible for a number of autonomic functions such as:

• Breathing
• Conduction Pathway for Nerve Tracts
• Digestion
• Heart Rate
• Swallowing
• Sneezing
• Defecation

     The medulla oblongata is continuous with the spinal chord and part of what is often known collectively as the brain stem. The medulla is so vital to life that diseases effecting it are often fatal.

    2. Lower part of the 4th ventricle

FIG.4

     The fourth ventricle is part of a network of cavities (the ventricular network funnily enough) The fourth ventricle runs between the sinus of sylvia and the obex and is filled with cerebrospinal fluid.

• The Metencephalon

1. Pons

FIG.5

    The Pons measures roughly an inch in length and lies between the Mid-brain and the Medulla Oblongata. It contains nuclei that deal with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture. It’s functions consist of:

• Relaying sensory information between the cerebrum and cerebellum reticular formation.
• Regulating awareness and sleep.

        2. Cerebellum

FIG.6

    Sometimes referred to as the “little brain” the Cerebellum is inferior to the cerebral hemispheres and has the appearance of being a separate entity to the rest of the brain. This is partially due to its surface appearance; in contrast to the cerebral hemispheres its surface is covered in closely spaced parallel grooves.  It is composed primarily of white matter with the exception of its surface of densely folded grey matter. The cerebellum is mainly involved in motor control; processing information from other areas of the brain, the spinal cord and sensory receptors to fine tune movement. Recent research suggests it also plays a role in the processing of certain temporal stimuli such as language and music. Functions:

• Fine movement coordination
• Balance and Equilibrium
• Muscle tone
• Basic memory and learning

        3. Rhombencephalic isthmus

     The Rhombencephalic isthmus is a constricted portion immediately adjoining the mid-brain and including the superior peduncles of the cerebellum, the anterior medullary velum, and the upper part of the fourth ventricle.

The MidBrain/Mesencephalon

    

FIG.7

     The Midbrain/Mesencephalon is superior to the Pons and inferior to the cerebral hemispheres. The dorsal portion of the midbrain is known as the tectum, it is responsible for reflexes relating to hearing and sight (e.g. the eye movement, pupil size, lens shape). The ventral portion of the midbrain is known as the tegmentum, it is essentially a complex network of neurones responsible for unconscious homeostatic and reflexive pathways. The midbrain also contains the crus cerebri which is composed of nerve fibres connecting the cerebellum and the cerebral hemispheres. Interestingly studies have shown that the midbrain of vertebrates may also be related to outbreaks of aggression.

The Forebrain/Prosencephalon

FIG. 8

     The Forebrain is superior to both the hindbrain and the midbrain as well as being the most anterior. It has major roles in  the following actions:

• Mastication
• Directs sensory impulses through the body
• Equilibrium
• Vision
• Eye movement
• Facial sensation
• Hearing
• Phonation
• Intelligence
• Memory
• Personality
• Respiration
• Salivation
• Swallowing
• Smell
• Taste

     The Forebrain is divided into two primary structures:

• Telencephalon (Cerebrum)

1. Cerebral cortex

FIG.9

    The cerebral cortex is the folded outer portion of the brain, in humans it is between 2-4mm thick. Its has the highest levels of uninsulated grey matter of any area of the brain. The cortex forms forded bulges (thus significantly increasing the area without increasing the volume) called gyri; so much so that more than 2 thirds of the brain lie in these folds (know as sucli). As well as being divided into portions covering the right and left hemispheres, the cerebral cortex can be subdivided according to differences in lamination:

a. Isocortex/Neocortex/Neopallium

    The isocortex is the outer layer of the cerebral cortex. It contains the most highly evolved stratification and organisation of any part of the human brain. It is made up of 6 layers of differentiated neurone cells of specified orientation. It should be noted that some functional areas of the isocortex are exceptions, lacking specific layers. The isocortex is (evolutionarily speaking) the newest area of the human brain and as such is the site of the majority of higher functions of the brain.

   The isocortex is divided into a number of topographically distinct areas or lobes. Each lobe is associated with particular processes.

i. Frontal lobe

FIG.10

    The Frontal lobe is the most anterior of the lobes and is additionally superior to the temporal lobe. This area of the brain is associated with many of the traits associated with personality (e.g ability to comprehend future results of actions), learning, impulse control, and prioritising actions. It is host to most of the brain’s dopamine receptors (these are the primary feedback through which learning is rewarded). This part of the brain is associated with emotion and subsequent modification in order to fit established social conventions.

ii. Temporal lobes

FIG. 11

   The temporal lobes are inferior to the frontal and parietal lobe and anterior to the occipital lobe. Studies suggest they are the primary part of the brain involved in declarative memory; damage to the temporal lobes can result in an inability to form memory after the event (anterograde amnesia). They contain the hippocampus (long-term memory) and are involved with auditory and higher visual perception (e.g. facial recognition).

iii. Parietal lobe

FIG. 12

    The parietal lobe is anterior of the occipital lobe, posterior of the frontal lobe and superior of the temporal lobes. The border between the frontal lobe and the parietal lobe is marked by the central sulcus. The border between the occipital lobe and the parietal lobe is marked by the parieto-occipito sulcus and the border between the temporal lobe and the parietal lobe is marked by the lateral sulcus. The parietal lobe coordinates information from multiple senses in order to establish spatial orientation. This is an area responsible for a number of higher cognitive processes such as speech and visual perception. It is also the area where the sensations of pain and touch are processed.

iv. Occipital lobe

FIG.13

      The Occipital lobe is the most posterior of all the main lobes of the brain. Anatomically this region contains most of the visual cortex (Brodmann area 17) and damage to the occipital lobes results in homonomous vision loss (i.e. the effect is the same in both eyes). The occipital lobes are where shape, colour, and like the temporal lobes, facial recognition take place. Projections from the occipital lobe to the superior temporal-parietal area are important for perceiving motion of objects.

b. Allocortex/heterogenetic cortex

    The Allocortex has fewer layers than the isocortex and is considered to be the “ancient” part of the brain. Allocortex structures include the olfactory cortex (concerned with the sense of smell) and the hippocampus (or archicortex).

c. Paleocortex

    An obsolete definition of this term exists describing paleocortex as an intermediate between allocortex and isocortex structures. This definition has not changed significantly except that nowadays the olfactory bulb is considered to be paleocortex.

2. Basal ganglia/nuclei

FIG.14

     The basal ganglia are a component of the corpus striatum and are in essence a set of interconnected nuclei within the brain. Information from the cerebral cortex passes to the basal ganglia where it is processed and then relayed back via the thalamus. There are a plethora of connections and pathways within and although the basal ganglia have long been implicated in motor function; it is known this is not there sole function, though the exact action in relation to behaviour control have yet to be properly established. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems. One theory suggests the basal ganglia decides which out of a number of possible actions the cortex may be planning, actually gets executed. Fitting this with idea that dopamine is used as a reward system for learning. The main components of the basal ganglia are:

1. Striatum - This area in particular is thought to play a critical role in memory and learning.
2. Pallidum
3. Substantia nigra (functionally related)
4. Subthalamic nucleus (functionally related)
5. Caudate nucleus
6. Lenticular/leniform nucleus 

• Diencephalon

FIG.15

    The diencephalon relays information between many different parts of the brain and controls a number of autonomic functions in the peripheral nervous system. It is also know to work in conjunction with the limbic system to generate and manage emotions as well with the endocrine and nervous systems. The diencephalon is involved in the following functions:

• Directing Sense Impulses Throughout the Body
• Autonomic Function Control
• Endocrine Function Control
• Motor Function Control
• Homeostasis
• Hearing, Vision, Smell, and Taste
• Touch Perception

  Structures that make up the diencephalon include:

1. Hypothalmus

     This structure works with the pituitary gland and the peripheral nervous system in maintaining homeostasis.

       2. Thalamus

     This is a limbic structure and is involved in perception/regulation of motor functions.  As a regulator of sensory information, the thalamus also controls sleep and awake states of consciousness.

        3. Epithalamus

      This structure is primarily concerned with connecting the limbic system to other parts of the brain. It contains the pineal gland which uses produces melotonin as part of its role in the modulation of sleep patterns.

FIG.1, 2, 4, 5, 6, 7,8,9,10,11,12,13- Adapted from Wikimedia

FIG. 3 – Wikimedia

FIG. 14 – Public domain

FIG. 15 – Wikimedia

The Biggest, Smallest, Longest, and Strongest Skeletal Muscles in the Human Body

The skeletal muscles are the striped, voluntary, powerfully and quickly contracting muscles. They are near the skin and are quite bulky.

The Body's Bulkiest Muscle

The biggest (bulkiest) of the skeletal muscles is the Gluteus maximus - the buttock muscle. It is a very superficial and prominent muscle. In humans it is bigger than that of chimpanzees and gorillas. There are three major muscles in the buttock: the Gluteus maximus, medius and minimus.
The Gluteus maximus has an origin in the iliac crest (the hip bones upper margin) as well as from the sacrum (the triangular bone forming the hip girdle). The Gluteus maximus is inserted into the femur, or thigh bone. The job of the Gluteus maximus is to move the thigh away from body in median line. It also rotates the thigh laterally, or sideways.

The Muscle With the Most Surface Area

The muscle with largest surface area is the latissimus dorsi, the broad muscle that covers the middle portion of the back. The word 'latissimus' itself means 'widest'. It originates from the last six thoracic vertebrae, lumbar vertebrae and last four ribs. It is inserted into the arm bone humerus with a tendon about 7 cm (slightly less than 3 in) long.

It helps in moving the arm toward the central axis of the body and also in arm rotation. Our shoulders can be drawn backwards and downwards with this muscle. It also helps in forced expiration in violent breathing movements, such as coughing or sneezing.

The Tiniest Muscle

The smallest skeletal muscle in the body is in the ear. It is the stapedius, attached to the smallest bone in the body, the stapes. The stapedius measures 1/20th of an inch (about 1.25 mm). This muscle helps to move the bones in middle ear that form the delicate hearing apparatus, and helps protect our sensitive inner ear.

The Longest Muscle

The body's longest muscle is the sartorius. It is a strap-like, narrow muscle which runs from the hip to the knee. It has its origin in upper part of ilium, the ear-shaped hip bone, and is inserted into the tibia, the broad, strong bone of the shank. The insertion resembles a flattened tendon. The sartorius moves our leg nearer the body in midline and helps to rotate and cross the leg.

The Srongest Muscle

It is claimed that the strongest muscle, in terms of force applied per unit area, is the masseter, the chewer muscle. Pressures to the tune of 122 kg or 270 lbs have been mentioned for force of bite of human molars. Jaw pressures of some 165 kg have been recorded by some reserachers, but not in humans - the data comes from dogs gnawing on bones.

The copyright of the article The Biggest, Smallest, Longest, and Strongest Skeletal Muscles in the Human Body in Human Anatomy is owned by Narayan Dattatray Wadadekar. Permission to republish The Biggest, Smallest, Longest, and Strongest Skeletal Muscles in the Human Body in print or online must be granted by the author in writing.