214 STRUCTURE OF THE KIDNEY. 



lution in water. To accomplish tins object, a special mechanism, the 

 kidney, is introduced. 



From this manner of considering the functional duty of the kidney, it 

 is very clear that a special relation must exist between this excreting or- 

 gan and the respiratory mechanism, for in the case of animals which 

 breathe by gills, or in those which, though subsequently atmospheric 

 breathers, receive their supply of aerated blood before birth by a placenta, 

 the conditions under which aeration takes place are such as permit the 

 removal of solid material by the respiratory mechanism. The urinary 

 excreting apparatus of an animal breathing air is therefore necessarily 

 burdened with an exclusive duty, which is shared by the gills and the 

 skin in a water-breather. 



In fishes, the renal apparatus is constructed under the condition here 

 The kidnev in indicated, an d though in many it appears to be greatly de- 

 birds, fishes, veloped, extending as a tubular arrangement from the skull 



cts, etc. through the abdominal cavity, it is to be regarded as analo- 

 gous to theWollfian bodies rather than to the true kidney. In reptiles 

 the proper kidneys appear ; in birds they are well developed, but their 

 secretion is, for the most part, a semi-solid substance, chiefly urate of 

 ammonia. The tubular form is presented in both insects and arachni- 

 dans, discharging its secretion into a cloaca. 



In man the kidneys may be described as a pair of dark-red ovoid bod- 

 The kidneys in i es ? placed one on each side of the vertebral column, in the 

 man - lumbar region, the right kidney being a little lower than the 



left. In the adult the kidney is four or five inches in length, and is en- 

 veloped in a mass of fat. Blood is brought from the aorta to supply the 

 organ by the renal or emulgent artery, and is carried back by the emul- 

 gent vein into the inferior vena cava. During its passage through the 

 kidney there is removed from the Wood a liquid secretion, the urine, 

 which*, flowing down a long channel, the ureter, is emptied into the blad- 

 der, from which it may be periodically removed. 



The supra-renal capsules are bodies of a yellow-red color placed above 

 Supra-renal the kidneys. They are much larger in the foetus than in the 

 capsules. adult, and doubtless have a reference to the peculiar conditions 

 of respiration obtaining at that time, for, as we have just observed, the 

 renal and respiratory mechanisms are necessarily interconnected. 



The substance of the kidney is described as consisting of two por- 

 Minute strnc- ^^ ons tne cortical and the medullary or tubular, as seen in 

 ture of the kid- Fig. 88, in which 1 is the supra-renal capsule ; 2, the vascu- 

 lar portion of the kidney; 3, 3, tubular portion grouped into 

 cones ; 4, 4, papillae projecting into calices ; 5, 5, 5, the three infundi- 

 bula ; 6, the pelvis ; 7, the ureter. (Wilson.) From which it appears 

 that the cortical substance is the external portion, and the tubular is 





