CIRCULATION. 



643 



Reptiles. The structure and functions of 

 the circulatory organs in Reptiles form a sub- 

 ject of great interest on account of the nume- 

 rous varieties which they exhibit in different 

 orders and genera, for in this respect the class 

 of Reptiles may be said to present to us an 

 anatomical analysis of the circulatory and re- 

 spiratory organs, and to constitute a gradually 

 simplifying series of forms, the observation of 

 which enables us to trace in the most clear and 

 interesting manner an analogy and correspon- 

 dence between the forms of these organs in 

 warm-blooded animals and in fishes, which, 

 but for the study of their structure in reptiles, 

 must very probably ever have remained hidden 

 from our view. 



In Fishes the heart consists of one auricle 

 and one ventricle, and a single current of blood 

 only passes through it. The structure of the 

 heart is very similar in some of the Batrachia 

 breathing by gills, but among other reptiles, 

 we find a gradual transition in the form and 

 structure of the heart from that just mentioned 

 as peculiar to animals with aquatic respiration, 

 to the double heart possessed by warm-blooded 

 and air-breathing animals. 



Among the Reptiles provided with lungs and 

 breathing air, some, as the Sauria, Ophidia, and 

 Chelonia, have the ventricular part of the heart 

 partially divided into two cavities (Jig- 314, 



Fig. 314. 



Heart of Lacerta ocellata. 



H, H'J which correspond in structure, relative 

 situation, and connections to the right and left 

 ventricles of the heart of warm-blooded ver- 

 tebrata; the anterior or right compartment 

 (H'} giving off chiefly the pulmonary (P), the 

 left or posterior ( H), the systemic arteries (A). 

 In the others, viz. the Batrachia and Protean 

 reptiles, the ventricle forms a single cavity 

 (figs. 317 and 318, H), and gives origin to 

 one large artery only (A), so that the pulmo- 

 nary and systemic arteries derive their blood 

 from the same trunk. In all of these, how- 

 ever, the auricle is double,* so that the venous 



* The auricle of the Batrachia was generally de- 

 scribed as single until the discovery of the left or 

 pulmonary auricle in the Frog and Toad by Dr. 

 John Davy. Mr. Owen has shewn this to be 

 the case also in the Newt and. Protean Reptiles ; 



blood from the system and the arterial blood 

 from the lungs are received into separate auri- 

 cular compartments of the heart, and are sub- 

 sequently mingled together in the common 

 ventricular cavity. In the Heart of the Croco- 

 dile of the Nile, Cuvier* has described three 

 compartments, one of which corresponds to the 

 left, the other two to the right ventricle, the 

 septum between the right and left sides being 

 incomplete. The heart of the Crocodilus Lu- 

 cius is described by Hentz, Meckel,f and 

 others as consisting of two ventricles, between 

 which the septum is quite complete, so as to 

 permit of no direct passage of fluid from one 

 side to the other, possessing therefore in this 

 respect, the same structure as the heart of warm- 

 blooded animals. In those of the above-men- 

 tioned reptiles in which the septum is so nearly 

 complete as to divide the ventricle into two 

 separate compartments communicating by a 

 small orifice, the arterial and venous blood are 

 believed to be kept separate from one another 

 by a valvular apparatus. Among the rest of the 

 Saurian, Ophidian, and Chelonian Reptiles, in 



Fig. 315. 



Heart of Common Tortoise. 



all of which the septum of the ventricular part 

 is less complete than in the Crocodile, there is 

 considerable variety in the extent to which the 

 division of the cavity is effected by the septum. 

 In a few of them the septum projects so little 

 into the ventricular cavity that it cannot be 

 supposed to divide to any extent, or to prevent 

 the complete mixture of the two kinds of blood 

 propelled from the opposite auricles. 



In the Crocodile, and in those Reptiles in 

 which the ventricular septum is nearly com- 

 plete, the circulation, so far as regards the 

 heart at least, may be considered as almost 

 double, or the same as in warm-blooded ani- 



Zool. Trans, 1834, p. 213. See also Martin St. 

 Ange's Plate of the Circulation, and M. Weber, 

 Bt'itr. zur Anat. und Physiol. Bonn, 1832. 



* Lemons, vol. iv. p. 221. 



t Vergleich. Anatomic, vol. v. p. 231. 



