Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/474

Rh 456 REPTILES [ANATOMY. continuous horizontal sheets of fibres in the ventral region, while for a greater or less extent longitudinal bundles of fibres underlie the vertebral bodies in different parts of the neck and trunk, and prolongations backwards of the dorsal and ventral muscles clothe the skeleton of the caudal region. Muscles like the temporal, masseter, pterygoid, and digastric of Mammals help to open and shut the jaws of Reptiles. Others descend from the dorsal region of the spine to the pectoral and pelvic girdles and to the limb bones, which latter are furnished with flexors and exten- sors, abductors and adductors, in essentially the same way as are the limbs of birds. There are two special deviations from this more normal type of structure. We find one of these in the Serpents, which, being limbless and without a neck, and requiring an extraordinary mobility in the parts of the axial skele- ton, have the muscles which clothe the trunk raised to their highest degree of multiplication and differentiation. The other exceptional type is furnished us by the Chelonians, which, having a perfectly rigid body and long neck and limbs singularly situated, have the muscles of the trunk atrophied to an extreme degree, the neck richly supplied with muscles, and also the limbs, while the limb muscles are so strangely different, in either origin or inser- tion, from those of other animals that their true nature and correct designations do not ssem as yet to admit of precise and accurate determination. luscles The Muscles of the Trunk and Tail. In Saurians the caudal f trunk muscles often take the form of a series of hollow cones successively ad tail; enclosed one within the other, while their continuations forward in the trunk become the longissirnus dorsi, sacro-lumbalis, spinalis, levatores costarum, and the other muscles familiar to anatomists. Subvertebral continuations form muscles which proceed from the vertebral bodies to the inner surfaces of the ribs, which they tend to pull back retrahentes costarum and further forwards, such muscles as answer to the longus colli, rectus capitis, &c., of Mammals. Ventral muscles take the form of the two obliques and transversalis, but the differentiation may be greater than in Mammals, as e.g., in Iguana, where the obliquus externus consists of three distinct layers. Tendinous or harder structures may produce a segmentation in the abdominal muscles, which, under special names, are prolonged to the hyoidian cornua and mandible. In Serpents the dorsal muscles are essentially the same as in Lizards, but are more developed, while small superficial muscles run from the ribs to the inside of the abdominal shields which are agents of locomotion. The limbless Serpent is practically some- what like a Centipede, and moves by the successive protrusion and retraction of each pair of ribs, which serve as two feet. For each pair is attached to a ventral shield the edge of which is applied to and removed from the ground by the ribs attached to it, according as these are protruded or retracted the shields thus serving as a number of small levers to propel the body along. The subvertebral muscle may also be very largely developed, as in Naja and Crotalus, the long hypapophyses of which have been already noted, and which serve for the attachment of such muscles. By their contraction the force of the downward flow of the poison fangs is greatly increased. That subvertebral series already mentioned as the retrahentes costarum is very extensively developed in the Ophidia. In the Crocodtliathe arrangement is substantially as in Saurians, but the so-called abdominal ribs induce certain differences. Some muscles lie above, some below, and some between these "ribs." Subvertebral muscles extend beneath the cervical and anterior dorsal vertebrae, and farther back there are retrahentes costarum. Hattcria has special muscles inserted into the distal expansions of its ribs, which may sometimes aid its motions in a very 'subordi- nate degree, as those of Serpents are so helped in the highest degree. 1 In the Chelonians the dorsal muscles are well developed in the neck and tail. Bifold muscles pass from within the hindmost lateral plates of the carapace to the dorsum of the caudal vertebrae. Other muscles extend between the articular and transverse pro- cesses of the caudal vertebrae. Dorsal muscles are entirely wanting in the thorax of Tcstudo, but in JSmys and Chclj/dra a longitudinal muscle lies upon the transverse processes of the trunk vertebrae. In the neck there are a number of muscles extending with various degrees of complexity between the transverse ami articular pro- cesses. The neck is retracted partly by a muscle passing from it to the procoracoid and partly by a long muscle passing to the 1 See Phil. Trans., pt. ii. for 1867, p. 17. head and neck from a greater or less number of the spines and neural arches of the more posterior trunk vertebra 1. Pyramidal muscles pass from the pelvis to the plastron, and oblique muscles arise from inside the costal marginal plates of the carapace anil pass to the pelvis and plastron, and a transversalis extends from within the costal plates and also passes to the plastron. A dia- phragm springs from the bodies of certain dorsal vertebrae and one or more ribs, and invests the surface of the lungs. The Muscles of the Head. In ordinary Saurians, the temporal of he;. ; muscle is embraced by the upper (in them the only) zygomatic arch, and descends from the side of the head to the coronoid process of the mandible. The masseter takes origin from the quadrate and coluiuella. Of the two pterygoid muscles one arises from the uppr.r surface of the pterygoid bone and the other from the os transversum. The digastric descends from the hinder part of the cranium to the posterior end of the mandible. In the Ophidia the muscular apparatus is much more complex, as might be predicted from the great mobility of their jaws. There are on each side three elevators and several depressors of the lower jaw, and sometimes, as, e.g., in Trigonoccphalus, a muscular belly connected with one of the elevators is so arranged as to be able to compress the poison gland. In poisonous Serpents also a tendon goes from the external pterygoid to the short maxilla, so that the latter can be erected by its intervention. The suspensorium is drawn upwards and backwards by a muscle arising from the neural spines of the anterior trunk vertebrae, and downwards and forwards by another arising from the basisphenoid. Four other muscles oil either side of the head are devoted to moving the palatine apparatus. The Crocodilia have a temporalis, an internal and external Eterygoid, and a digastric. There are also a pair of sternomandi- ular muscles, and of hyomandibulars (from the cornua of the hyoid), a pair of genioglossi, and a superficial mylohyoid. The Chclonia have only an internal pterygoid, besides the temporal and digastric. But they have also a geniohyoid and genioglossus as well as a mylohyoid. The Muscles of the Pectoral Girdle and Limb. In ordinary of pe Saurians and in Crocodiles the scapula is drawn forwards and toral upwards by a trapezius and levator, and there are also a serratus girdle magnus and a sternoscapular muscle, a cleidomastoid or sterno- and mastoid, and generally an omohyoid. In the Chclonia the lira shoulder-girdle muscles are very reduced and peculiar, and no muscle draws it towards the skull, but a muscle arises from the sides of the cervical vertebrae and is inserted into the scapula. Another (smaller) passes from the second dorsal transverse pro- cess and the part of the carapace therewith connected to the scapula, which it draws backwards. Another muscle goes from the coracoid to the hyoid, and yet another a sort of sternomastoid from the plastron to the skull. Lacertilians have an ordinary pectoralis and generally a subjacent small muscle passing from the coracoid to the great tuberosity of the humerus. There is a latissimus dorsi going to the lesser tuberosity and a large subscapularis. A triceps ends by a tendon inserted into the olecranon, often containing an ossicle analogous to a patella. There is also a biceps, coracobrachialis and brachialis, anticus, with other Mammalian muscles, flexors, and extensors of the digits, with even lumbricnl muscles, but not with that perfect arrangement of perforating and perforated tendons which character- izes the highest class. The muscles of the pectoral limb of Crocodiles, though showing many minor differences from those of Saurians, yet present a fundamental general resemblance to the latter. It is otherwise with Chelonians. In them the pectoralis major is represented by two muscles, one a muscle extending from tlie median plastron plate and external border of the carapace to the inner of the two tubcrosities of the humerus, and another muscle inserted beside the former and taking rise from the most anterior plate of the plastron. There is also a superior pectoralis, which arises from the coracoid and coracoacromial ligaments, and which is partly inserted into the same tubcrosity and partly extends beyond it. There are also a coracobrachialis and a deltoid, but no snprascapular. All the muscles which arise from the scapula are inserted into the external tuberosity of the humerus. There is a muscle which represents the latissimus dorsi. It arises beneath the most anterior plate of the carapace, and is inserted into the external tuberosity. The muscles of the forearm and extremity are less exceptional. Muscles of the Pelvic Girdle and Limb. In Lacertilians there are of several hind-lirnb muscles which have a subcaudal origin; some of gii these end at the thigh, while others extend to the feet. There arc and numerous flexors of the leg. A powerful muscle arises beneath the liml tail, and is inserted by a broad tendon into the femur. From the inferior margin of that tendon another slender tendon arises, sometimes ending (as, e.g., in Iguana) in the interarticular cartilage of the knee-joint. The muscles of the thigh, leg, and foot have a general similarity to those of Mammals generally, as have those of the arm and hand, but those which represent the ham-string muscles have generally much complexity and intricacy of arrangement of