CHAP. XXXIV.] KIDNEY. 483 



these glands is not determined in the Crustacea ; but among the Aracknida, 

 tubes composed of basement-membrane and containing epithelium, exist. 

 Guanin also has been detected in them, so that there can be little doubt of 

 their real nature. Among Insects, renal organs exist as long narrow tubes, 

 and the presence of uric acid has been detected in several species. In the 

 Mottusca, except in the lowest class, kidneys are distinctly observed, and are 

 either two in number, or combined to form a single organ with an excretory 

 duct. The spongy organs of the Cephalopoda have been proved to be true 

 kidneys, and uric acid has been detected in them with the murexide test by 

 E. Harless. Kidneys exist throughout the vertebrate classes, and are com- 

 posed of tubular glands, provided with one or more efferent ducts, connected 

 with which are often observed numerous appendages. The uriniferous tubule 

 consists essentially of a tortuous tube of basement-membrane, lined with 

 secreting epithelium, and dilated at its closed extremity, so as to embrace a 

 tuft of highly tortuous capillary vessels. 



The specific gravity of the healthy kidney is about 1-050, but is 

 liable to vary somewhat, according to the quantity of fluid which 

 exists in the organ at the time of examination. 



The following is an analysis of the cortical portion of a healthy 

 human kidney by Dr. Beale. The organ was taken from the body 

 of a healthy man, thirty-one years of age, who was killed by falling 

 from a second-floor window : 



100 parts of 

 Solid Matter. 



Water . . . . 76-450 



Solid matter . . . .... . 23-550 



Fatty matter, containing much cholesterine . -939 3*98 



Extractive matter, soluble in water . . 5-840 24-79 



Fixed alkaline salts . . . ,, ; ,./, , . I'OIO 4-28 



Earthy salts -396 1-68 



Albumen vessels, etc 15-365 65-24 



Even in health, the proportion of water and solid matter varies 

 greatly, which fully accounts for the varying statements of different 

 observers with regard to the weight of the healthy kidney. In 

 disease, the composition of the secreting structure of the kidney 

 undergoes great alteration. The fat is very much increased in 

 quantity in kidneys in a state of fatty degeneration. The relative 

 proportion of the solids generally may be much diminished in 

 quantity, which is remarkably the case in some specimens of en- 

 larged kidney. The increase of size, in these instances, being ac- 

 counted for by an unusual quantity of water in the tissue of the 

 organ ; but in many cases, it is, no doubt, dependent upon deposi- 

 tion of new matter. In a very large kidney, weighing half a pound, 



