RESPIRATORY CIRCULATION IN REPTILES. 



19' 



357 H 



augment the impetus with which the blood is sent into the 

 branchial arteries. 



The circulation in Reptiles is not double, like that of 

 fishes; for only a part of the blood is brought under the in- 

 fluence of the air in the pulmonary organs. All the animals 

 belonging to this class are cold-blooded, sluggish, and inert; 

 they subsist upon a scanty allowance of food, and are as- 

 tonishingly tenacious of life. The simplest form in which we 

 meet with this* mode of circulation is in the Batrachia; it is 

 shown in the diagram, Fig. 357. The heart of the Frog, 

 for example, may be considered as consisting of a single 

 ventricle (E,) and a single auricle (D.*) 

 From the former there proceeds one 

 great arterial trunk, which is proper- 

 ly the aorta. This aorta soon divides 

 into two trunks, which, after sending 

 branches to the head and neck, bend 

 downwards (as it is seen at o, p,) and 

 unite to form a single trunk (A,) which 

 is the descending aorta. From this 

 vessel proceed all the arteries which 

 are distributed to the trunk and to the 

 limbs, and which are represented as 

 situated at B: these arterial ramifica- 

 tions are continued into the great ve- 

 nous trunks, which, as usual, constitute the venae cavae (c,) 

 and terminate in the auricle (D.) 



From each of the trunks which arise from the primary 

 division of the aorta, there proceed the small arteries (F,) 

 which are distributed to the lungs (H,) and convey to those 

 organs a part only of the mass of circulating blood. To 



* Dr. Davy has observed that although the auricle appears single, when 

 viewed externally, its cavity is in reality divided into two compartments 

 by a transparent membranous partition in which some muscular fibres are 

 apparent: these communicate with the cavity of the ventricle by a common 

 opening provided with three semilunar valves. Edin. Phil. Journal; xix. 

 161. 





