THE

ANATOMY OF VERTEBRATES.

:

CHAPTER XXVII.

MUSCULAR SYSTEM OF MAMMALIA.

THE muscular tissue in the present as in the preceding Vertebrate classes presents the two conditions of striped and unstriped elementary fibres the striped kind, comprising all the voluntary muscles with those of the heart, are red: deeper coloured in Cetacea and Carnivora than in Ungulata: deeper in the pectoral muscles of Cheiroptera than in those of the legs: paler in the pectorals and other muscles of the fore-legs of the Kangaroo than in the psoæ and those of the hind-legs: palest in some Rodentia.

§ 192. The Diaphragm.-The chief characteristic of mammalian myology is the diaphragm, vol. ii., fig. 139, d, which, as such, is not more completely developed in Man than in the Monotreme. It is the partition between the thoracic and abdominal cavities, fig. 1, vaulted and convex toward the thorax, fig. 2, and consists of carneous and tendinous

verging to be inserted into the external ligamentum arcuatum,'

fig. 1, d, and into the aponeurosis called centrum tendineum' or cordiform tendon,' ib., T. This centre is widely notched toward the spine, and divided anteriorly into three tracts, of which the right is usually the largest. Between the right and middle tracts is the orifice, c, for the inferior vena cava (postcaval' of Mammals). Behind the tendon, and to the left of the median line, is the orifice, e, for the oesophagus and pneumogastric nerves: the

2

aorta, a, passes from the chest to the abdomen between the crura' of the lesser muscle. The right 'crus' in Man arises from the three or four upper lumbar vertebræ; the left crus does not descend so low: both muscular bundles expand as they rise, decussate at the œsophageal opening, and are inserted into the posterior concavity of the central tendon and internal ligamentum arcuatum, fig. 1, f.

The diaphragm is most muscular, longest, and most oblique in Cetacea, in which the central tendon is almost obsolete: by rising so far back, it permits the proportional extension of the lungs, which in the Dugong and Manatee act as air-bladders. In the perissodactyle Ungulates, in which the moveable ribs are numerous and continued to near the pelvis, the diaphragm is also extensive, and much arched toward the thorax.

§ 193. Muscles of Monotremata.-To give an account of the muscular, as fully as that of the osseous, system of the Mammalia, would not be attended with the same advantages, even if a detailed myology comported with the scope and extent of the present work. This part of Mammalian anatomy will therefore be limited to the notice of a few select examples. Fig. 3, from Meckel,' shows the more remarkable muscles of the Ornithorhynchus. The animal is dissected from the ventral surface; the great panniculus carnosus,' 1, is reflected from the right side, and the deeper-seated muscles are shown on the left. The panniculus carnosus, which is remarkable for its thickness, encompasses nearly the whole body, adhering most firmly to the external skin, but separated from the subjacent muscles, especially where it covers the thorax, abdomen, the arm,

and the thigh, by a copious and lax cellular tissue; and in the female, at the abdominal region, by the mammary glands. The fibres are chiefly longitudinal, but at the lower part of the neck become transverse. The obtuse posterior end of the muscle is attached by three or four fasciculi to the dorsal aspect of the caudal diapophyses. The legs and the arms protrude through oblique apertures in this muscular tunic; some of the anterior fasciculi are inserted by a short tendon into the pectoral ridge of the humerus ; and others, still more anterior, are attached to the cranium, the lower jaw, and lower lip. A strip of fibres, which is cut off at 1*, is attached to the os hyoides; another fasciculus (1′) spreads over the cheek-pouch, F, and assists in emptying that receptacle of the food.

The trapezius, 9, is divided into two muscles; the posterior portion is an oblong slender triangle arising by a broad tendon from the tenth and eleventh vertebræ and ribs, and inserted by a short strong tendon behind the extremity of the spine of the scapula ; the anterior portion arises from the occiput and tendinous raphé connecting it with its fellow of the opposite side, and is inserted into the spine of the scapula, and into the outer half of the clavicle.

The latissimus dorsi, a very long and broad muscle, arises from the spines of all the dorsal and lumbar vertebræ and from the eleven posterior ribs; it is inserted by a broad and strong tendon into the distal half of the ulnar margin of the humerus, and, with part of the 'panniculus,' into the fascia attached to the olecranon and spreading over the fore-arm. At its anterior part this muscle may be separated into a superficial and deep stratum. The rhomboideus is a single muscle, but thick and long, inserted into the narrow base of the scapula.

The splenius capitis is united by an intermediate tendon with the opposite muscle, and is inserted into the mastoid process.

The biventer cervicis and the complexus are distinct throughout their whole course, which extends from the anterior dorsal and posterior cervical spines to the occiput; the complexus is the longest and thickest muscle, and divides into an external, shorter, and deeper-seated portion, and an internal, longer and superficial portion.

The sacrolumbalis arises from the dorsal extremity of the ilium, is attached to the ribs, over which it passes in its course to its insertion into the transverse processes of the four or five posterior cervical vertebræ : it is continued by the cervicalis ascendens' to the atlas.

The longissimus dorsi is a much thicker and narrower muscle,

and extends from the dorsal aspect of the sacrum along the spine to the third or fourth cervical vertebra. It is continued forward by the transversalis cervicis and trachelo-mastoideus, which are blended into a single oblong muscle arising from the anterior dorsal and inserted into the transverse processes of the six lower cervical vertebræ and the mastoid process.

The sterno-mastoid is a double muscle on both sides, one portion being superficial, s, the other deep-seated; each arises separately from the episternum, and is separately inserted into the mastoid. The omo-hyoideus, 10, and mylo-hyoideus, 10, have a common insertion into the hyoid. A muscle, 1", arising from the basi-hyal and expanding to be inserted into the lower lip, serves to retract this part. The sterno-hyoideus, 11, joins the hyoglossus. The genio-hyoideus, 12, and the stylo-hyoideus, 13, have the normal relations: the biventer maxilla, 14, is a short thick muscle, inserted near the bend, representing the angle, of the jaw.

The caudal muscles are powerfully developed. The oblique fibres of the inferior or deflector muscles are shown at 53; they are removed on the other side to expose the anterior caudal nerves, i. The obliquus externus abdominis, 3, 3, arises from all the vertebral ribs, except the first, and from the dilated extremity of the ilium; it is inserted by a strong tendon into the outer extremity of the marsupial bone, VI, then expands into an aponeurosis which is attached to the internal margin and base of that bone, and into the symphysis pubis, decussating with the tendinous fibres of the opposite muscle: it does not split to form an abdominal ring.'

The obliquus internus, 6, arises from the anterior part of the ilium, expands, and is inserted into the broad cartilages of the seven posterior ribs, v, v.

The transversus abdominis, 7, is a thicker muscle, and arises from both the ilium and the lumbar diapophyses; its tendon passes behind the recti to blend with that of the opposite muscle, and with the aponeurosis of the obliqui externi, in the linea alba.

The pyramidalis, or superficial rectus, 4, is here, as in the ordinary Marsupials, of very large size; it arises from the whole inner margin of the marsupial bone; its fibres converge toward and are confluent at the linea alba with those of its fellow, and it gradually terminates in a point opposite the posterior part of the sternum. It depresses the ribs, shortens the abdomen, and protracts the marsupial bone.

The rectus abdominis, or posterior rectus, 5, arises from the posterior margin of the marsupial bone, and is inserted into the