OCR Text |
Show 4 REPTILIA. form of a Fish respiring by branchire, to that of a Quadruped breathing by lungs. Some of them, however, always retain their branchire, and a few have never more than two feet.(l) ORDER I. CHELONIA. The Chelonia, better known by the name of Tortoises, have a heart composed of two auricles, and of a ventricle divided in two unequal cavities, which communicate with each other. The blood from the body is poured into the right auricle, that from the lungs into the left, but the two streams become more or less mingled in passing through the ventricle. These animals are distinguished at the first glance by the double shield in which the body is enveloped, and which allows no part to project except their head, neck, tail, and four feet. The shell (or upper shield) is formed by the ribs, of which there are eight pair, widened and reunited by denticulated sutures, and with plates adhering to the annular portion of the dorsal vertebra, so that all these parts are rendered fixed and immovable. The inferior shell is formed of pieces, usually nine in number, ana1ogous to a sternum.(2) A frame composed of bony pieces, which have been considered as possessing some analogy with the sternal or cartilaginous portion ( 1) The Sauria and Ophidia are differently arranged by some others, Merrem, for instance. They detach the Ct•ocodiles, to form a separate order, and unite the first family of the Ophidia or Anguis to the remainder of the Sauria, a distribution which is founded on some peculiarities in the organization of Crocodiles, and on a certain resemblance of Anguis to the Lizards. We merely indicate these affinities, which are almost wholly internal, preferring a division more easily applied. (2) See Geoff. An. du Mus. t. XIV, p. 5; and on the entire osteology of the Tortoises, my Hech. sur les oss. foss. tom. V, 2e partie. [N.B. It is well tore· mark, that I shall hereafter designate the upper shell by the single word shell (tuta) and the inferior by sternum. .11m. Ed.] · . CHELONIA. 5 of the ribs, and which in one subgenus always remains in a cartilaginous state, surrounds the shell, uniting and binding together all the ribs which compose it. The vertebrre of the neck and tail are consequently the only ones which are movable. These two bony envelopes being immediately covered by the -skin or by plates, the scapulre and all the muscles of the arm and neck, instead of being connected with the ribs and spine, as in other animals, are attached ben~at~ : the same arrangement is found in the bones of the pelv1s and all the muscles of the thigh, so that in this respect the Tortoise may be -said to be an inverted animal. The vertebral extremity of the scapula is articulated with the sh~ll ; and the opposite limit, which may be considered analogous to a clavicle, is joined to the sternum. So that the two shoulders form a ring through which pass the resophagus and trachea. A third bony branch, larger than either of the others, and directed downwards and backwards, represents, as in Birds, the coracoid apophysis, but its posterior extremity is free. The lungs have considerable extent, and are situated in the 'Same cavity with the other viscera.(l) The thorax, in most of them, being immovable, it is by the play of its mouth that the Tortoise respires, which it effects by keeping the jaws closed, and alternately raising and depressing the os hyoides. The former of these motions permits air to enter through the nostrils, the tongue then closes the internal orifice of those apertures, when the latter forces the air into the lungs.(2) Tortoises have no teeth; their jaws are invested with horn like those of Birds; the Chelydes excepted, where they are covered with skin only. Their tympanum and palatine arches {1) Observe that in all those reptiles in which the lung penetrates into the .abdomen fand the Crocodile is the only one in which it does not) it is enveloped hke the int~stines by a fold of the peritoneum, which separates it from the abdominal cavity. (2) With respect to this mechanism, which is common to Tortoises and to the Batrachians, see the Mem. of Uobert Townson, Lond. 1779. |