90 SUPPLEMENT ON 



veins which pass along the internal border of the leaflet, 

 and both open into the greater vein of the arch, so that these 

 carry back the blood which the arteries have brought for- 

 ward. But in passing out of the dorsal part of the branchiae, 

 the branchial veins assume the structure and functions of 

 arteries ; even before their arrival at this point, the anterior 

 send several branches to different parts of the head, and it is 

 proper to remark, that the heart and several parts lying in the 

 breast receive blood from a branchial vein, by means of an 

 offset issuing nearly from its source, and therefore much an- 

 terior to its exit from the branchiae ; nevertheless, it is only 

 when the trunks of the four branchiae are united, that the 

 great artery is formed, which supplies the viscera and parts 

 of the body, and is therefore the artery which represents the 

 aorta of mammiferae, although it is destitute at its basis of 

 both ventricle and auricle. For the left cavities of the heart 

 of mammalia, we here find substituted a simple vascular 

 apparatus, situated above the branchiae, while the right cavi- 

 ties, which are properly represented, lie beneath them. 



The aorta already noticed gives off at the beginning a large 

 branch for the viscera, variously distributed in different spe- 

 cies, to the liver, the stomach, intestines, spleen, genitals, 

 and swim-bladder ; and after feeding, by means of other 

 branches, the kidneys, the intervals between the ribs, and the 

 muscles of the trunk in general, it passes on into the annuli, 

 beneath the caudal vertebrae, and follows the direction of the 

 spine. The blood distributed into the head, the trunk, the 

 branchial apparatus, the genitals, and swim-bladder, returns 

 towards the heart by the great venous sinus ; but with the 

 exception of some ramifications, that of the stomach, intes- 

 tines, and spleen, passes to the liver by the vena portae, which 

 is a communication liable to be greatly diversified in different 

 species of fishes. 



Respiration. The blood of fishes is exposed to the infill- 



