. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE CIRCULATION OP 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
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE CIRCULATION OP 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. Evolntion. — 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- 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. Fig. Veins of the toot of the horse (afte
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