496 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



distribute the arterial blood to the S3^stem at large ; whence it is again 

 returned by the venous system, after passing through the systemic 

 capillaries to the auricle (Fig. 184). In fishes, therefore, the respira- 

 tory apparatus forms a part of the general systemic circulation, the 

 heart being, therefore, a branchial and not a systemic organ, and the cir- 

 culation being simple instead of imperfectly double, as in the reptilia, or 

 perfectly double, as in the warm-blooded vertebrata. In the fish, a 

 portal S3 r stem, composed, as in all vertebrates, of veins from the digestive 

 apparatus, conducts the blood from the abdominal organs through the 

 kidneys and liver ; hence, in the fish, both these glands receive venous 

 blood (Fig. 185). 



In the reptilia the heart consists of two auricles and one ventricle 

 (Figs. 186 and 187). The right auricle receives venous blood from the 



FIG. 188. DIAGRAM OF THE CIRCULATION 



IN REPTILIA. (Beclard.) FIG. 189. DIAGRAM OF THE CIRCULATION 

 P, lungs; O, left auricle; V, ventricle, whence the IN THE REPTILIA. (Carpenter.} 



blood is driven through the systemic circulation to enter A, ventricle; B, left auricle ; C, right auricle : D, pulmo- 

 the right auricle, O', after being collected by the veins. nary circulation; E, systemic circulation. 



system at large; the left auricle receives arterial blood from the lungs; 

 both discharge their contents into the single ventricle, which thus re- 

 ceives a mixture of venous and arterial blood. From the ventricle the 

 blood is driven partly through the lungs and partly to the general 

 S3 r stem, so both lungs and system receive a partially aerated blood, 

 forming an incomplete double circulation (Fig. 188 and 189). In 

 the reptilia, as a rule, there is a distinct arterial and distinct pul- 

 monary trunk arising from the ventricle, but in the amphibia there is 

 only a single trunk, of which the pulmonary arteries are branches, rising 

 from the ventricle. In the crocodile there exists a partial ventricular 

 septum, so placed that it serves to direct the dark venous blood entering 

 from the right auricle chiefly into the pulmonary arteries, whilst the 

 arterial blood coming from the left auricle is sent out into the systemic 



