THE CIRCULATION OF THE BLOOD. 285 
sloth, that hangs from trees; in the legs of swans, geese, etc. ;in 
the horse’s foot, in which the arteries break up into many small 
‘divisions. It has been 
suggested that these ar- 
rangements 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 as the space it trav- 
erses, in all probability 
serves the same purpose. 
Evolution. — The com- 
parative sketch we have 
given of the vascular sys- 
tem will doubtless sug- 
gest a gradual evolution. 
We observe throughout a ° 
dependence and resem- 
blance which we think 
can not be otherwise ex- F's. 247.—Veins of the Lal ® the horse (after Chau- 
plained. The similarity 
of the foetal circulation in the mammal to the permanent circu- 
lation of lower groups has much meaning. Even in the high- 
- est form of heart the original pulsatile tube is not lost. The 
great veins still contract in the mammal; the sinus venosus is 
probably the result of blending and expansion. The later 
differentiations of the parts of the heart are clearly related to 
the adaptation to altered surroundings. Such is seen in the 
foetal heart and circulation, and has probably been the deter- 
mining 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 
be less under nervous control, but it certainly is least depend- 
ent on the nervous system, and has the greatest automaticity. 
It is surely fortunate for man that this part of the reptilian 
heart is represented in his own. In cases of fainting, partial 
drowning, or other instances of impending death, this part, with 
