REPTILIA. 



309 



shortly after its origin into three branches, of 

 which one is the right or systemic aorta, 

 the second the right subclavian, and the third 

 the common trunk of the carotid and left 

 subclavian arteries. The left or splanchanic 

 aorta, previous to dividing among the vis- 

 cera, gives off a large branch which communi- 

 cates with the right descending or systemic 

 aorta. The three great arteries, viz. the 

 pulmonary, and right and left aorta, are 

 closely connected together immediately after 

 their origin, and dilate into expansions which 

 are collectively larger than the cavities of the 

 heart. In the common state of circulation 

 the blood passes from the right ventricle 

 chiefly into the pulmonary artery, and partly, 

 also, into the branch arising from it, to enter 

 into the left aorta. The blood of the left 

 ventricle, on the other hand, is thrown into 

 the right aorta, right subclavian, and carotid 

 arteries, a small quantity only passing into 

 the left aorta. When the animal is under 

 water, the action of the lungs being inter- 

 rupted, and the circulation of blood through 

 them suspended, a larger proportion of the 

 contents of the right ventricle must pass into 

 the branch of communication with the left 

 aorta, and it is probable that under such cir- 

 cumstances only does it happen that the blood 

 sent to the various organs is an admixture of 

 arterial and venous blood, as in the Chelonia 

 and other Sauria. 



In the serpent (Python Tigris, Dane?.) the 

 blood of the general system is collected into 

 a large elongated sinus, formed by the union 

 of the inferior with the right superior cava. 

 The left superior cava winds round the back 

 of the left auricle, receives the coronary veins, 

 and terminates in the lower part of the orifice 

 which leads from the above sinus to the right 

 auricle. This orifice is protected by two 

 elongated semilnnar valves. The whole of 

 the inner surface of the auricle, with the ex- 

 ception of these valves and the opposite valve 

 of the foramen ovale, is reticulated with 

 delicate muscular fasciculi. The left auricle 

 receives the blood from the lungs by a single 

 pulmonary vein, and has a similar muscular 

 structure : there is no valve at the termination 

 of the vein in this auricle. The blood enters 

 the posterior or aortic division of the ven- 

 tricle by two crescentic apertures, which are 

 each provided with a single semilunar valve, 

 extended from either side of the septum of 

 the auricle. The fleshy septum, which ex- 

 tends from the base of the ventricle to the 

 space between the roots of the pulmonary 

 and systematic arteries is incomplete at its 

 upper and anterior part, and leaves there a 

 free communication between the pulmonary 

 and aortic chambers : these also intercommu- 

 nicate by several round apertures of different 

 sizes near the apex of the ventricle, which 

 serve to thoroughly blend together the two 

 kinds of blood, before they are expelled, thus 

 mixed, along the three arteries which scpa* 

 rately arise from the ventricles. The origins 

 of the pulmonary artery and left aorta are 

 each provided with a pair of semilunar valves. 



The carotid arteries are given off from the 

 right aorta, \\luch afterwards unites with the 

 left aorta at some distance below the heart. 

 Nervous System. The brain of reptiles, in 

 Fig. 223. 



Anatomy of the Brain of Turtle. {After Swan.*) 



a. 1, corpus striatum and a lesser oblong eminence 

 seen on opening the lateral ventricle ; on the left 

 side the choroid plexus is seen passing through an 

 opening in the septum, to communicate with that 

 of the right side ; part of the striated body has been 

 removed on the right side ; 2, thalamus of the optic 

 nerve ; 3, optic lobe and ventricle continued forward 

 under the thalami, forming a resemblance of the 

 third ventricle, and then backwards into the cere- 

 bellum, and to the calamus scriptorius. 



b. l,cut surface, from which the striated body has 

 been removed ; it is the cms of the brain, and is 

 somewhat connected with the commissure of the 

 optic nerves : the thalamus on this side has been 

 cut off at its connection with the optic tract. 2, 

 Optic lobe, from which more has been removed than 

 in the preceding figure. 3, Cerebellum, from which 

 more has been removed than in a ; two longitudinal 

 bands are continued on from the base of the optic 

 lobes, and terminate near the calamus scriptorius, 

 by being implanted into the anterior portion of 

 the oblong medulla : 011 each side of these, others les 

 distinct may be observed. 4, fourth nerve. 



the completeness of its structure, occupies a 

 position intermediate between that of birds and 

 fishes ; it resembles the former in the smooth- 

 ness of its surface and the small size of the 

 optic thalami, and the latter in the length of 

 the olfactory lobes, and their continuity with 

 the anterior extremity of the hemisphere ; its 

 proportionate size relative to the dimensions 

 of the body is, however, far inferior to that of 

 birds, although it still completely fills the 

 cranial cavity. 



The olfactory lobe of the brain is hollow, 

 and its cavity communicates with the ven- 

 tricle contained in the cerebral hemisphere. 

 Each hemisphere, as in birds, consists of a 

 central portion, or corpus striatum, the re- 

 lative size of which varies in different orders, 

 and of a nervous expansion which incloses 

 the ventricle above and on its inner side. 



The optic thalami are small, and occupy 

 their usual position on each side of the third 

 ventricle. The tubercnla quadrigemina are 

 situated as ordinarily above the aqueduct : 

 they are of a rounded form, and, as in birds, 

 contain a ventricular cavity, which is in com- 

 munication with the third ventricle. The 

 anterior and posterior commissures of the 



x 3 



