268 



COMPARATIVE PHYSIOLOGY. 



permit of a supply of arterial blood being maintained without 

 congestion of the parts. Very marked tortuosity of vessels, as 

 in the seal, the carotid of which is said to be forty times as long 



Fig. 232. — Section of a lymphatic rete mimftife, from the popliteal space (after Chan- 

 vean). a, a, afferent veeaelfi. ft, ft, efferent vessels. The whole very strongly sug- 

 gests a crude form of lymphatic gland. 



as the space it traverses, in aU probability serves the same pur- 

 pose. 



Evolution. — The comparative sketch we have given of the 

 vascular system will doubtless suggest a gradual evolution. We 

 observe throughout a dependence and resemblance which we 

 think can not be otherwise explained. The similarity of the 

 foetal circulation in the mammal to the permanent circula- 

 tion of lower groups has much meaning. Even in the highest 

 form of heart the original pulsatile tube is not lost. The great 

 veins still contract in the mammal ; the sinus venosus is proba- 

 bly the result of blending and expansion. The later differentia- 

 tions of the parts of the heart are clearly related to the adapta- 

 tion to altered surroundings. Such is seen in the foetal heart 

 and circulation, and has probably been the determining cause 

 of the forms which the circulatory organs have assumed: 



It is a fact that the part of the heart that survives the long- 

 est under adverse conditions is that which bears the stamp of 

 greatest ancestral antiquity. It (the sinus venosus) may not 



