LAB 5 & 6

EXTERNAL FEATURES AND THE MUSCULAR SYSTEM

 
Objectives: 1) Be able to identify the three different muscle types based on their microscopic anatomy. 2) Understand how eye, hypobranchial, axial and appendicular muscles form from embryonic myotomes and how branchiomeric muscles form from a visceral sheet of hypomere. 3) Know the external features of the three animals dissected in this lab. 4) Use the dogfish to learn the eye muscles and the nerves that innervate them. 5) Use the rat/cat to learn the abdominal muscles 6) For each animal, learn all the muscles that are covered in this lab manual. INTRODUCTION You will be forming groups of 6 to dissect the three animals, the cartilaginous dogfish shark, the amphibian mud puppy Necturus, and a mammal, the laboratory rat or domestic cat. These animals are compared in the remaining labs. Not every animal will be equally easy to dissect and some labs will spend more time on one animal than another. You will have to work as a team to spread the load and make sure that you all have equal time to learn the material. Discuss your progress as you are going along, what your approach will be and how to modify it if something is more complex or time consuming than you thought. This lab examines the external features and muscular system of three animals. Although we will be only looking at one type of muscle, that of the skeleton, it is important to note that there are three types of muscles in the body and they are all displayed in the lab under microscopes. Look at the demonstration slides and wall charts of smooth, skeletal (or striated) and cardiac muscle. These muscles are classified based on their cellular structure and membrane properties. Smooth muscle is usually from hypomere mesoderm or embryonic mesenchyme and forms the visceral or involuntary muscles such as blood vessels and gut. They have no striations, a central nuclei and spindle-shaped fibers that taper and dovetail together. Skeletal muscle is sometimes called striated muscle due to its striated appearance when viewed under the microscope. It is normally from myotome, except for the branchiomeric muscles in the head, which are from undifferientated somitomeres. They are voluntary muscles and the muscle is striated (banded), the many nuclei are peripheral and the fibers are long, cylindrical and unbranched. Cardiac muscle, found in the heart, is striated, (banded) but involuntary. They are mononucleated, separated by intercalated discs. They have branching strands and come from hypomere. Make sketches of each muscle type smooth striated cardiac Muscles can only contract (shorten). Relaxation is the lack of contraction. Muscles have origins (fixed part of muscle attachment) and insertions (attachment site that moves relative to the origin). If you pull on a muscle with your forceps you can usually see the origins (anchor) and insertions. They often work in pairs called agonists and antagonists. Some examples are abductors (away from midline) and adductors (towards midline), flexors (reduce angle) and extensors ((increase angle), protractors (move limb forward) and retractors (move limb backward), levators (lift) and depressors (lower). Deeper muscles may insert by faschia or by tendons onto bone. Most of these are of dense collagen fibers. Ligaments, which are also composed of collagen fibers, connect bone to bone. We will be studying the muscles of the skeleton and eye in this lab. These muscles come from embryonic myotomes, which are segmented, in the embryonic body into somites. In the head region, they are reduced and may not be segmented, and are referred to as somitomeres. The first three segments in the head region are located in front of the ear (pre-otic). The epimere mesoderm in these segments survives as the muscles of the eyes; the hypomere mesoderm does not develop. Each segment has split to form different eye muscle masses, which have then moved apart. We can follow these masses because their innervation (the nerve going to them) remains the same. We will look in detail at the eye muscles of the shark, but every vertebrate, including man, has a similar pattern of eye muscles and nerve innervation. The mesodermal somitomeres in the post-otic head region do not contain a coelom and instead form a solid sheet called branchiomere. This branchiomeric muscle becomes the muscles of the jaw, hyoid arch and gills. It is innervated by cranial nerves (somatic sensory, visceral sensory and visceral motor nerves, all from the dorsal roots). These dorsal nerves in the first three segments are the nerves passing through the orbit of the eye to serve the branchiomeric muscles of the head, jaws and spiracle in the dogfish. More posterior, they work the gills. They perform a similar function in Necturus, but in mammals, with the loss of gills, the more posterior muscles are lost or transformed into larynx muscles and the trapezius muscles of the shoulder. Branchiomeric muscles also form the smooth muscles of the viscera and gut, but these muscles will not be studied in this lab. Another group of muscles supplying the jaw are the hypobranchial muscles. These muscles are formed from myotomes of somitomeres behind the gills that move (as embryonic mesenchyme) forward and downward to the ventral region of the pharynx (between the gills). These, along with the eye muscles, form the axial muscles of the head. The axial muscles of the trunk are from mesodermal somites and form the skeletal muscles of the trunk and tail. The segmented origin of these muscles can be easily traced in the dogfish and mud puppy but becomes obscured by massive limb muscles in the mammal. Appendicular muscles are formed in the shark by muscle buds in the embryonic myotomes. They serve to operate the pelvic and pectoral fins. The axial muscles of the body do much of the shark locomotion. The advent of limbs in Necturus, with elbow/knee and wrist/ankle joints makes limb musculature far more complex. In mammals, limbs have become more powerful for increasing speed on land and appendicular muscles lie over many of the body (axial) muscles. They begin as dorsal (abductors) and ventral (adductor) muscle groups and connect parts of limbs or the limbs to the trunk. Although derived from axial muscles, they are usually considered separately. DOGFISH EXTERNAL ANATOMY Note the overall shape and design of the dogfish and how it is designed for swimming. Feel the placoid scales and note they point backward. Examine the fins - 2 dorsal (with spines), caudal (note shape of lobes), pectoral (1 pair), pelvic (1 pair) - as well as the heterocercal tail. Determine the sex of your animal by noting if the pelvic fins bear claspers, found in the male. To help you decide, look at the plastic blocks showing male and female pelvic fins. Follow the lateral line along each side of body. It is used for to sense vibrations and movements in the water; interconnected neuromast cells relay pressure waves. male female The head has a sub terminal mouth, a pair of lidless eyes, a pair of spiracles, 5 pairs of gill slits, and a pair of nares. Examine the surface of the head. Note the continuation of the lateral line onto the skin of the head in three principal lines, the supra orbital, the sub-orbital and the mandibular. Look at the ampullae of Lorenzini, fluid filled pits that sense low-level electrical stimulus and sense depth. Find the two endolymphatic ducts, which connect to the inner ear, which is for balance only. Examine the cloaca, a chamber between the pelvic fins and note the urinary papilla, and anus. Dissection: Skin the left dorsal side of the head from the snout to about 4 cm in front of the first dorsal fin. Use your scalpel or scissors to cut through the very thin but tough skin. The skin is closely adhering to the muscles, so be patient. Use a blunt probe and hold the muscles to separate the skin from the muscles; a scalpel will only cut the muscles. When you get to the eye, you will have to cut through the skin surrounding the eye and dorsal cartilage that forms a crest over the orbit. Remove the gelatinous packing material and blood behind the eye with blunt forceps. You should identify the eye muscles in situ from above and below the eye, taking care not to break the delicate nerves or muscles in the orbit. DOGFISH MUSCLES Extrinsic Muscles of the Eye (colour yellow) The eye muscle dissection is concerned with the first three pre-otic segments. In these segments the three preotic myotomes (somitomeres) form the six muscles of the eye. These muscles are innervated by somatic motor nerves. The origin of the eye muscles is the cartilaginous wall of the orbit. The insertion is attached to a more freely movable part, the eyeball. Before you dissect around the eye, look at the dissected heads, in plastic cases, to see where the muscles are located. Pull the eye gently away from the orbit in order to expose the six muscles, which connect the eye to the orbit. These muscles can be divided into two groups: the two oblique muscles which have their origins very close together in the anterior medial corner of the orbit and the four rectus muscles which have their origins in the posterior medial corner of the orbit. It is important to identify these muscles correctly as they are supplied by cranial nerves that are encountered later. Superior oblique muscle: (s.o.) Origin: anterior medial corner of orbit Insertion: dorsal surface of eyeball. Innervation: trochlear nerve (IV) 2) Superior rectus muscle: (s.r.) Origin: posterior medial corner of orbit. Insertion: dorsal surface of eyeball. Innervation: occulomotor nerve (III) These two muscles insert side by side on the dorsal surface of the eye. Their fibers run at right angles to each other, the superior oblique anteriorly and the superior rectus posteriorly. Be careful not to damage the fine trochlear nerve, which innervates the superior oblique muscle. It runs superficially from the muscle to the median wall of the orbit. 3) Lateral (External) rectus muscle: (p.r.) Origin: posterior medial corner of orbit. Insertion: dorsal posterior surface of eyeball. Innervation: abducens nerve (VI) This is the only muscle on the dorsal posterior surface of the eye. Its fibers run medially from the point of insertion on the eyeball. 4) Inferior rectus muscle: (i.r.) Origin: posterior medial corner of orbit. Insertion: mid-ventral surface of eyeball. Innervation: occulomotor nerve (III) 5) Inferior oblique muscle: (i.o.) Origin: anterior medial corner of orbit. Insertion: mid-ventral surface of eyeball. Innervation: occulomotor nerve (III) These inferior muscles can be seen by tilting the eyeball upwards and looking on its ventral surface. They insert side by side, their fibers running at right angles to each other. The origin of this muscle is right beside that of the superior oblique muscle. Be careful not to confuse the inferior oblique muscle, which runs from the eyeball to the medial wall of the orbit, with the optic nerve. 6) Medial (Internal) rectus muscle: (a.r.) Origin: posterior medial corner of orbit. Insertion: medial surface of eyeball, near its anterior end. Innervation: occulomotor nerve (III) This muscle can best be seen by looking from the dorsal surface, between the eyeball and the medial wall of the orbit. Its insertion is beneath the superior oblique muscle. Its fibers run posteriorly. Dogfish Eye This is a good chance for us to take a look at the eye. It is similar in all vertebrates except the lens is rounder in water animals than those on land due to differing optics. The outer, white layer is the sclera. In front it becomes a clear, transparent cornea. Cut around the eye to remove the cornea and open up the eye. Do not remove it from the dogfish. Expose the round lens in the vitreous cavity. In life it is clear. The middle layer has three parts. The posterior, black, choroid layer is vascular, the ciliary body with suspensory ligaments hold and move the lens and the coloured iris directs the light to its pupil opening. The innermost, whitish layer is the retina, which has rods (black and white) and cones (colour). Most vertebrates see in colour except for mammals where only the higher primates have colour vision. The eye is held in place by a cartilaginous optic pedicle resting on the sclera. This is not to be confused with the optic peg, which is attached to the jaw. Dogfish Branchiomeric Muscles (colour red) Key: po-preorbital, c-cucullaris, cb-coracoid bar, cc-common coracoarcual, ch-coracohyoid, cm-coracomandibularis, dc - dorsal constrictors, e- extensors (abductors), f-flexors (adductors), la- linea alba, im-intermandibularis, lh- levator hyomandibulae, vc - ventral constrictors, vlb, ventral longitudinal (hypaxial) bundle. The 4-7 gill slits are served by nerves X -XI and are branchiomeric muscles: levators (cucullaris, interarcuals) and superficial constrictors. Behind the gill arches, the somitomeres grow down and forward as hypobranchial muscle which fills the ventral area between the gill arches with coracoid muscles and is innervated by the hypobranchial nerve XII. Posterior to these muscles we have typical myotomes forming axial muscles and special appendicular muscles for the pectoral and pelvic limbs. The first arch contains the jaw muscles. The adductor mandibulae, a large muscle below the spiracle, closes the jaw. The intermandibularis is on the ventral surface, posterior to the jaw. It originates on the Meckel's cartilage (lower jaw) and inserts on the central raphe. Its function is to elevate the floor of the mouth for swallowing. In the hyoid arch, the levator hyomandibulae can be located posterior to the spiracle where it compresses the gill pouches. The interhyoideus is another ventral constrictor muscle, but it is hidden by the intermandibularis. In the gill arches, there are a variety of muscles to operate the gills. The levator muscle of note is the cucullaris, lying above the gills, which results from a fusion of all the levators and serves to elevate the scapula and gill arches. Other muscles (constrictors, adductors and interarcuals) are located deep within and between the gills. Only the hyoid and superficial constrictors can be seen. Dogfish Axial Muscles (colour yellow) Key: dlb - dorsal longitudinal bundle (epaxial), hs - holizontal septum, llb - lateral longitudinal (hypaxial) bundle, mm- myomere, ms - myosepta, vlb - ventral longitudinal (hypaxial) bundle. These somatic (myotomal) muscles are the skeletal muscles of the trunk and tail. The anteriomost myotomes form hypobranchial muscles, which move anteriorly between the gills to form some of the muscles of the throat. Superficially, between the gills and attached to the scapulocoracoid bar can be seen the triangular coracoarcual muscles which open the mouth. The epaxial muscles are found dorsal to the horizontal septum and the hypaxial muscles are ventral to it. The epaxial muscles above the gills are called epibranchial. There is a myomere for each vertebra. The horizontal septum in anchored to the dorsal ribs and the ventral ribs develop in between, in the myosepta. The ventral hypaxial muscles meet at the midline forming a connective tissue raphe, the linea alba. Dogfish Appendicular Muscles (colour blue) Skin the left pectoral fin near its base and look for the dorsal extensors (elevators) that lift the fin, the pectoral abductor muscle. On the ventral side are the flexors (depressors) that pull it down, the pectoral adductor muscle. Necturus External Anatomy male female The flattened head has a mouth with lips, small lidless eyes and external nares. There are three pairs of lateral gills and two pairs of gill slits. The head is separated from the trunk by a gular fold. In the head and trunk regions small indents of the lateral line system can be seen. In males, the cloaca is surrounded by a large cloacal gland. The skin aids in respiration and has a thin stratum corneum with mucous and poison glands. Dissection: Remove the skin on the entire left side of the body. Begin by making a dorsal incision along the back, with scissors. Do not go very deep or you will cut into the muscles. Cut around the external gills and leave them in place. Carefully pull skin off each appendage, leaving it inside out, like removing a sweater. Necturus Eye Muscles These muscles are the same as those of the dogfish and will not be studied in Necturus. Necturus Branchiomeric Muscles (colour red) Key: bh-branchiohyoideus, cb-coracobranchialis, dl-(dilator laryngis), dm - (depressor mandibulae), dt - dorsalis trunci, ds - dorsalis scapulae, e- (extensors), gh - (geniohyoid), ha - humeroantebrachialis, ih-interhyoideus, im -intermandibularis, la-levatores arcuum, ld - latissimus dorsi, lm - levator mandibulae = masseter and temporalis, oa- (omoarcual), p- pectoralis, pc - procoracohumeralis, ra - rectus abdominis Controlling the mandibular arch are the temporalis and masseter muscles. They form a sling for the lower jaw to close the mouth. On the ventral side lies the intermandibularis, extending from the mandible to the median raphe. It elevates the throat. For the hyoid arch, the branchiohyoideus can be found lateral and posterior to the jaw. It waves the gills back and forth in the water for respiration. Ventrally, posterior to the intermandibularius, lies the interhyoideus, which elevates the throat. The posterior portion of this muscle is the sphincter coli which becomes most of the facial muscles in mammals. In the gill arches, directly above the gills, lie the levatores arcuum and cucularis (trapezius), which elevate the gills. Most of the other gill constrictor muscles have disappeared or moved to the larynx. Necturus Axial Muscles (colour yellow) Key: cb-ccoracobranchialis, eo-external oblique, ha- humeroantebrachialis, ih-interhyoideus, im- intermandibularis, la - linea alba, ms - myotomal segment, p - pectoralis, pc- procoracohumeralis, ra- rectus abdominus, rc- rectus cervicus, sc-subracorcoideus Anteriorly, the hypobranchial muscles can be seen in the center of the throat, behind the gular fold. Their segmentation can be seen in the rectus cervicis, which retracts the hyoid and gills and depresses the head. The pectoriscapularis pulls the shoulder towards the gills. The dorsal epaxial and ventral hypaxial muscles are separated by the horizontal septa of the myotomes are similar to those of the dogfish. Dorsally the epaxial muscles are fused into a long, strap-like dorsalis trunci. Ventrally the hypaxial muscles meet at the linea alba. Lateral to the linea alba lie the strap-like longitudinal rectus abdominis muscles. There are three layers of abdominal muscles, the external oblique, internal oblique and transversus abdominis, all of which will be observed in the mammal. On either side of the cloaca are located the ischiocaudalis muscles and external to them, the caudocrualis muscles which serve to flex the tail. Necturus Appendicular Muscles (colour blue) Pectoral muscles developed from the fin muscles of fish. The dorsal, pectoral abductor, muscles will be examined first. Note the latissimus dorsi, a large triangular muscle originating on the faschia of the dorsalis trunchi and inserting on the humerus, which is used to retract the limb. The trapezius (cucullaris) also originated from the dorsalis trunchi and inserts on the scapula to pull it anteriorly. This muscle is branchiomeric in origin and not an appendicular muscle. The dorsalis scapulae originates on the superscapular cartilage and inserts on the humerus to pull it anteriorly. It is related to the deltoid muscles of mammals. Laterally lies the procoracohumeralis, which pulls the humerus anteriorly. The triceps brachii lie on the dorsal surface of the upper arm with three heads from the coracoid, scapula and humerus. They insert together on the ulna and serve to extend the forearm. The ventral, pectoral adductor muscle has become large, ventral pectoral muscle masses that adduct (move inward) the humerus. The anterior muscle, the supracoracoid originates on the coracoid cartilage of the pectoral girdle. The pectoralis muscles originate on the linea alba. The upper forearm is flexed by the humero-antebrachialis, which originates on the humerus and inserts on the radius and the coracobrachialis, which originates on the coracoid process and inserts on the humerus. The distal forelimb has dorsal extensors homologous with the shark abductors, which extend the limb. It also has ventral flexors homologous to adductors to flex the limb. Key: cc-caudocrualis, cf-caudofemoralis, dt -dorsalis trunci, eo- external oblique, ie-ilioextensoralis, it-iliotibialis, p-i-f- pubo-ischio-femoralis internis, pt- pubotibialis The pelvic dorsal abductors have also been highly modified. They have given rise to small muscles on the anterior of the limb. The iliotibialis and the hidden iliofibularis, which form the sartorius and gluteus muscles of mammals, originate on the ilium and insert on the tibia and fibula to abduct the hind limb. The ilioextensoralis is visible in Necturus but hidden in mammals. The pelvic adductor muscle from the ventral surface of the shark fin becomes even more highly diversified in Necturus. On the ventral side, between the limbs, note the large anterior pubo-ischio-tibialis, which along with the pubo-tibialis adduct the hind limb. The more posterior ischio-flexorius flexes the shank and foot. The large pubo-ischio-femoralis and pubo-ischio-tibialis muscles originate on the ventral surface of the girdle and adduct the femur and tibia. The distal hind limb, like the forelimb, has dorsal extensors to extend the shank and foot, which are homologues to the shark pelvic abductors. There are ventral flexors, which adduct and flex the lower leg and foot, which are homologues to the adductor of the shark pelvic fin. Rat/Cat External Anatomy  Note the skin with hair and the mammary glands, characteristic of mammals. The head has a mouth with lips. The upper lip is split by a groove, the philtrum. The nose has nares and the eyes bear upper and lower eyelids and a reduced nictitating membrane. The external ears (pinna) direct sound. The whiskers or vibrissae are sensory. The trunk has an anterior thorax and posterior abdomen. The anus is at the base of the tail. The urinary and genital openings are separate in females, but in males they run as one duct to the tip of the penis. Males have a double pouch, the scrotum containing the testis. In the female, between the skin and muscles lies granular, mammary gland tissue in a band under the lines of nipples.  Dissection: Cut through the skin of the rat/cat dorsally from the head and the full length of the back. Cut around the left eye and ear. Remove the skin from the left hand side of the animal. Skin the animal by inserting the handle of the scalpel or a blunt probe between the skin and the muscles. The muscles are connected to the skin by fascia and muscles attached to the fascia are used to twitch the skin. The cat will be much more difficult to skin than the rat and will probably have large fat areas under the skin which can be removed. Try and work quickly so you have time to view the muscles. Mammal Eye Muscles These muscles are the same as those of the dogfish and will not be studied in mammals. Look at the dissected eyeballs in display cases and human eye model. Mammal Branchiomeric Muscles (colour red) In the mandibular arch, the temporalis muscles lie from behind the eye to the ear, occupying the temporal fossa of the skull. It elevates the mandible (lower jaw). Behind and below the eye is the large masseter muscle, which elevates the jaw and allows for complex chewing. Ventrally and medially lies the digastric muscle, which depresses the mandible. In the hyoid arch, the most important muscle in mammals is the sphincter coli, which is the posterior portion of the interhyoideus muscle. This muscle spreads onto the neck and becomes the platysma in mammals such as the rat (not examined here). It then spreads over the head and face to form the facial muscles of higher mammals such as humans and allows them to have greater facial expression. The gills are now gone and the muscles of the gill arches have for the most part disappeared. Some muscles remain on the larynx and for swallowing. The cucullaris muscle has survived as the trapezius muscles of the pectoral girdle. The trapezius complex consists of three muscles, which originate on the spine. Two insert on the scapula to adduct and move it dorsally and forward. The third muscle inserts on the clavicle and protracts the humerus. The sternomastoideus muscle is also derived from the cucullaris muscle and serves to turn or flex the head. Mammal Axial Muscles (colour yellow) In mammals the hypobranchial muscles form the deep muscles of the throat and tongue. The sternohyoid can be seen on top of the larynx. Much of the primitive metamerism is lost due to the appearance of complex appendicular muscles for the movement of limbs and the development of broad muscular sheets of parietal muscles in the abdomen. The epaxial muscles have become long bundles to straighten and flex the body. These are deep muscles on either side of the spine and are hidden by the limb muscles. The hypaxial muscles are three sheets of muscles joined ventrally at the linea alba. It is formed by a muscle-to-muscle attachment of fascia called an aponeurosis. Cut out a 3 sided rectangle of abdominal muscle through to the body cavity, leaving the mid-line uncut. Pull apart the layers of muscle from the lateral side towards the mid-line. See diagram on next page. On either side of the linea alba is a band of longitudinal muscle, the rectus abdominis, where the three muscle layers sit in a common sheath. The three muscle layers from the outside to the inside are: external oblique, internal oblique, and transversus abdominus. These muscles support the abdominal viscera in a muscular sling. Other, deeper hypaxial muscles are not seen. These are the intercostals and supracostals of the rib cage and the diaphragm, which acts as a suction pump in respiration. Tail muscles are continuations of the epaxial and hypaxial muscles bundles. Abdominal Muscles, Ventral view Mammal Appendicular Muscles (colour blue) In the pectoral region, the pectoral abductor muscles of the dogfish now are represented by a number of forelimb muscles. Note the triangular latissimus dorsi, which now originates from the neural spines of most of the thoracic vertebrae and inserts on the humerus, to pull the forelimb dorsally. Anteriorly it is overlaid by the trapezius complex, which is branchiomeric in origin. From the scapula there are five deltoid muscles, which insert on the humerus to adduct and rotate it. Locate the spinodeltoideus. The triceps brachii consists of three muscles originating on the scapula or humerus and inserting together on a process of the ulna, to extend the forearm.  The ventral pectoral adductor muscles of the dogfish have also become complex in mammals. The biceps brachii is located on the anterior of the forearm. They originate on a tendon inserting on the scapula and insert by a tendon on the tuberosity of the radius, to flex the forearm. Turn the animal on its back to see the large pectoral muscles. There are several muscles in this group in mammals, some of them adduct or pull the forelimbs towards the midline, others retract the forelimb backwards. These muscles originate on the sternum and insert on different parts of the humerus. The most obvious of these muscles will be the anterior pectoralis major and the more posterior pectoralis minor. The pelvic abductor muscles of the dogfish are highly specialized for powerful locomotion in the mammal. In the thigh is found the large sartorius muscle, which runs from the ilium to the knee to adduct and rotate the femur and extend the shank. More posteriorly lay the thin, wide, gluteus muscle of the hip, originating on the last sacral and first caudal vertebrae and inserting on the femur to abduct the thigh. The pelvic adductor muscles now occupy both surfaces of the hind limb. Anterior on the limb is the tensor fascia latae, a triangular muscle originating on the ilium and inserting on the knee by a fascia lata. It extends the lower leg (shank). Posterior to it lies the caudofemoralis, partly hidden by the biceps femoralis. It originates on the tail (caudal) vertebrae and inserts on the femor. The posterior thigh has the large, thick biceps femoralis, which runs from the ischium to the tibia and knee to abduct the thigh and flex the shank. The posterior most muscle is the semitendinosus, which is stuck to the biceps femoralis and runs from the ischium to the tibia to flex the shank. Ventrally can be seen the large gracilis muscle which occupies the posterior half of the thigh. It runs from the pubic symphysis to the tibia and adducts and retracts the leg. Key: bf- biceps femoralis, cf-caudofemoralis, gm- gluteus maximus, st- semitendinosus, tfl - tensor fascia late Labs_5_&_6__Muscles_files/N-vent-post-jpg.