SANGUIFEROUS SYSTEM IN THE VERTEBRATE. 91 



vessels run in innumerable parallel branches on the anterior surface 

 of the coffin bone. Another peculiarity in this system is presented 

 by the inferior cava, in the porpoise, the seal, the common and the 

 sea otter, consisting in a considerable dilatation of this vessel 

 between the liver and the diaphragm, similar to what is observed 

 in tortoises and diving birds. 



A general review of the vascular system indicates that the heart 

 in its simplest form resembles a vessel endowed with contractility, 

 as exemplified by the vessel-like multiple hearts which constitute 

 the vascular trunks of the annelides, the contractile trunks on the 

 alimentary canal of the holofhuria, the dorsal vessel of insects, &c. 

 In the embryo of the highest warm-blooded animals the heart is at 

 first tubular, and it is interesting to observe, that during the progress 

 of its development it passes through, and resembles the several forms 

 which constitute its permanent type in the adult state of fishes and 

 reptiles, even the clefts in the neck, with the arched divisions of the 

 aortic trunk, which are persistent in reptiles, may be seen in the 

 human embryo at a very early stage of its development, and the 

 ductus arteriosus, which is single in mammalia, but double in birds, 

 is the last of those arches which remains unclosed in the foetus. 

 These arches may be well seen by inspecting the embryo of a bird 

 on the third day of incubation. 



The frequency of the heart's action varies much in different ani- 

 mals, and even in the human subject, from a variety of causes. 

 In a fish it beats in a minute from 20 to 24 

 In the frog, about - - 60 



In birds, from - - 100—140 



In the bat, - - 200 



In rabbits, - 120 



In the cat, - J 10 



In the dog, - - 95 



In the sheep, - 75 



And the horse, - 40 



In the human embryo, - 150 



At birth, - - 130 to 140 



During the first year - - 115 — 130 

 During the second year - 100 — 115 



During the third year, - 90 — 100 



During the seventh year, - 85 — 90 



About the fourteenth year - 80 — 85 



in the middle period of life, - 70 — 75 



In old age, - - 50—65 



RECAPITULATION. 



1. The systemic circulation was discovered by Harvey in 1619. 



2. In the higher radiata a large artery surrounds the beginning 

 of the alimentary canal, in the form of a ring, from which the 

 systemic branches arise. 



