654 PISCES. 



runs in the same groove as the artery, but is more deeply situated, and 

 moreover runs in the opposite direction : that is to say, the branchial 

 artery, derived from the heart, and coming from the ventral aspect 

 of the body, diminishes in size as it mounts towards the back, and gives 

 off twigs to the branchial fringe ; whereas the branchial vein, on the 

 contrary, receiving blood from the lamellaB of the branchia, increases in 

 diameter as it approaches the dorsal region. 



(1793.) On leaving the gills, the branchial veins assume the appear- 

 ance and perform the function of arteries. The anterior, even before 

 escaping from the branchial arch, gives off ramifications to different 

 parts of the head ; and the heart and parts adjacent likewise receive 

 their supply of arterial blood from a branchial vein. 



(1794.) The veins derived from all the branchial arches ultimately 

 unite and form the aorta, which evidently corresponds to the aorta of 

 Mammalia, although it has neither auricle nor ventricle at its com- 

 mencement. 



(1795.) The aorta, while in the abdomen, runs beneath the spine, and 

 gives arteries to the viscera in the usual manner ; but at the commence- 

 ment of the tail it becomes enclosed in the inferior vertebral arches, by 

 which it is defended to its termination. 



(1796.) There is yet another set of organs, which, as we ascend from 

 inferior to higher forms of animal life, we encounter for the first time in 

 the class before us an apparatus for elaborating the urinary secretion, 

 which is peculiar to the Vertebrate classes. 



(1797.) The kidneys in Fishes are very voluminous : they are situated 

 on each side of the mesial line, immediately beneath the bodies of the 

 vertebrae, and extend along the whole length of the abdomen, not un- 

 frequently reaching to the base of the skull, where their anterior por- 

 tion (fig. 317, e) lies above the branchial apparatus. The ureters (fig. 

 317, /) generally terminate in a kind of bladder-like dilatation, the ori- 

 fice of which is found behind that of the vulva (s). 



(1798.) Examined minutely, the substance of the kidney is found to 

 be entirely composed of microscopic tubules, which terminate in the 

 ureters : these uriniferous tubes are variously contorted, but of equable 

 diameter throughout ; and they end, towards the periphery of the kidney, 

 in blind extremities. 



(1799.) The skin of these aquatic animals is perpetually lubricated 

 by an abundant mucous secretion furnished by muciparous follicles, or 

 secreted in long tubular organs placed beneath the skin. In the Skate 

 the vessels last mentioned are remarkably large, and their distribution 

 very extensive. 



(1800.) The brain of an adult fish occupies but a small portion of 

 the cranial cavity, the space between the pia mater, which invests the 

 brain, and the dura mater, which lines the skull, being occupied by a 

 loose cellular tissue filled with fluid : there is consequently no serous or 



