198 A Manual of Veterinary Physiology. 



portion of it which excretes the water does not excrete all 

 the solids : the water is removed from the blood by the Mal- 

 pighian bodies, the solids are removed from the blood in the 

 tubules : in both cases the epithelial cells which line these 

 parts are the active factors in the matter. The Malpighian 

 body is arranged so that the blood is always at high 

 pressure within it : this is accomplished by the efferent 

 vessels being smaller than the afferent. At this high 

 pressure the watery part of the blood transudes through 

 the epithelium into the capsule surrounding the tuft 

 (known as Bowman's capsule), and thence into the 

 tubules. It has been stated that a certain proportion of 

 proteid also passes through the epithelial wall of the tuft, 

 to be reabsorbed later on in the tubules : but as proteids 

 do not occur in normal urine, we may safely believe that 

 the epithelium of the tuft does not in an undamaged con- 

 dition allow any to pass. During the passage of the water 

 through the tubule, the secretory cells of the latter pass 

 into it the organic and inorganic material which they have 

 separated from the capillary vessels which surround them, 

 and at the same time it is stated that a portion of the water, 

 of which an excess is excreted in the Malpighian tufts, is 

 reabsorbed. 



There are no known secretory nerves of the kidneys ; the 

 vessels are under the control of the vaso-motor system, and 

 depending upon Avhether they be dilated or constricted, so 

 we have an excess or diminished secretion, but no secretory 

 nerves proper are known. 



The Composition of the Urine depends upon the class of 

 animal. In all herbivora, with certain minor differences, 

 the urinary excretion is much the same, bul not so with 

 omnivora or carnivora, which possess, especially the latter. 

 a distinctive urine. When herbivorous animals live on 

 their own tissues, as during starvation, they become car- 

 nivora, and their urine alters completely in character, 

 corresponding to the urine of flesh feeders. The young oi 

 herbivora, if still sucking, also have a urine possessing 

 much the same properties as that of carnivora. 



