100 PHYSIOLOGY 



In the inferior orders of animals the kidneys are pretty much of the 

 same uniform substance through the whole ' ; but in the quadruped they 

 appear, when cut into, to be formed of two different substances, one 

 called the ' cortical,' from its being exterior, the other the ' tubular 2 .' 



The cortical substance has its distinguishing appearances from its 

 vessels running in all directions, having no particular direction of fibres, 

 and also having the cryptae interspersed everywhere through its sub- 

 stance. The other substance, or tubular, is placed towards the centre 

 of the kidney ; when cut in one direction it appears to be made up of 

 parts or fibres passing pretty parallel to one another towards the centre, 

 and when torn in that direction it splits into numberless fibrous parts. 

 This substance begins insensibly in the surrounding secretory part, and 

 passing inwards, they of course converge and terminate at once, forming 

 one side of a cavity, called the ' pelvis.' 



As the kidneys have another action of their blood-vessels superadded 

 to that of support, similar to every other secreting body, and which is 

 to dispose of some of the blood in secretion besides the nourishment to 

 the part itself, they are therefore endowed with supernumerary blood- 

 vessels for such purposes, and are of course extremely vascular when 

 compared to many other parts of the body. 



These vessels in Fish and upwards arise from the great artery, or aorta, 

 as that artery passes along the back-bone. In Fish this great artery is 

 giving off the arteries to the kidneys through the whole course of the 

 kidneys, therefore there are a vast number of small arteries going to 

 those bodies. In Amphibia and Fowl the kidneys are more collected 

 and of course their arteries are less numerous, and larger in proportion ; 

 but in the still more perfect animals [mammals], where the kidneys are 

 more circumscribed bodies, there we have in common only one artery 

 to each kidney, which is of a very considerable size. 



In those kidneys where the arteries go into them in small branches, 

 as in fish, &c, there is not that necessity for their very quick ramifica- 

 tions, for being originally small, they come soon to their ultimate 

 arteries ; but in the others, where the artery goes into the kidney by 

 one trunk, and therefore is large, it is obliged to ramify very quickly, 

 in order to form the ultimate arteries. 



As the arteries of the kidneys in Fish come to them in innumerable 

 small branches, and as the motion of the blood in those animals is slow 

 and languid, the arteries therefore appear to terminate in their ultimate 

 branches, as in other glands. But in the more perfect animals, especial- 

 ly the quadruped, — where the artery goes into the kidney in one short 



1 [Hunt. Prep. 1186.] a [lb. No. 1218.] 



