AND PHYSIOLOGY. 



237 



421. Heart of Fishes —The heart consists of one auricle 

 and one ventricle, "which are covered by a pericardium, and 

 the whole organ is very small in proportion to the size of the 

 whole body, being from T } } 0 th 

 to toV o tn i ts weight. In the 

 osseous fishes the heart is 

 elongated and conical, while 

 in the Sharks and Rays it is 

 broader. The ventricle dis- 

 charges its blood through the 

 aortic trunk upon the gills. 

 This trunk divides up into 

 a large number of minute 

 branches which ramify upon 

 the gills, and after the blood 

 has received its oxygen from 

 the water, it is collected by 

 a corresponding set of ves- 

 sels, and emptied into another 

 trunk which supplies all the 

 re3t of the body — which trunk 

 corresponds to the aorta — 

 though it has no muscular 

 power to propel the blood along. After it has performed its 

 office it is collected by a system of vessels similar to veins, 

 and returned to the auricle. 



422. Pulsations in a Minute. — Commonly not more than 

 twenty or thirty beats in a minute may be counted in fishes, 

 while in birds one hundred may be counted in the same 

 time. 



423. Portal Circulation —In fishes, as in reptiles, there 

 seems to be a double portal circulation. 



Circulation in Fishes, a, Heart. 

 Auricle, c, Ventricle. d, Circulation 

 through the Gills, or Lesser Circulation. 

 0r, Circulation through the Body, or 

 Greater Circulation, e, Arteries, f, Veins. 



421. Why is the heart of the osseous fishes called a branchial heart? What proportion 

 of the body does it constitute? Give the course of circulation. 422. What number of 

 pulsations can be counted in the heart of fishes, and what number in birds? 



