772 SPECIAL PHYSIOLOGY. 



cava. The lymphatics are numerous, and consist of a superficial and 

 deep set. The renal nerves are small but numerous, and may be 

 traced even on to the afferent arteries of the Malpighian corpuscles; 

 but their mode of termination is unknown. They are derived from 

 the sympathetic nerves found on the renal artery, and from the lesser 

 splanchnic nerve. 



The ureter, its pelvis and calyces, are composed of an external 

 fibro-elastic coat, continuous with the proper capsule of the kidney ; 

 of a muscular layer, which consists of external longitudinal and inter- 

 nal circular fibres; and of a mucous coat, continued on to the papillae, 

 and lined with a spheroidal epithelium. The ureters, in animals, con- 

 tract slowly on the application of galvanism or other stimuli ; some- 

 times they act rhythmically. Their lower ends enter at each side, 

 the fundus or base of the urinary bladder penetrating its coats ob- 

 liquely, and opening into it by a narrow elongated orifice, so guarded 

 by muscular bundles that the reflux of urine into them is prevented. 



Action of the Kidneys. 



The purpose of the enormous number of uriniferous tubes is, as in 

 glands generally, to increase the extent of secreting or excreting sur- 

 face, within a given space. It has been calculated that the total 

 amount of surface in the coiled tubes in each kidney exceeds forty- 

 four square feet ; and to this must be added the excreting surface of 

 the straight tubuli as well. (Vierordt.) The Malpighian bodies are 

 quite peculiar to the kidneys, these little vascular coils, projecting into 

 a duct, having no resemblance to the solid sacs which bear the same 

 name in the ductless gland, the spleen. So special a structure has 

 doubtless some special office. Since these bodies have been shown 

 to project into the tubuli, at or near their commencement, it has been 

 conjectured that they separate from the blood the greater part of the 

 water of the urine, and that then the inspissated blood which passes 

 from them by the efferent vessels enters the vascular plexus around the 

 tubuli, and yields to the spheroidal epithelium the proper solid con- 

 stituents of the urine, which are thence excreted in the tubules them- 

 selves, and are washed away out of those canals by the watery exuda- 

 tion descending from the Malpighian corpuscles. The naked condition 

 of the glomerulus, arid the squamous character of its epithelium, are 

 fitted for a simple process of transudation ; whilst the spheroidal 

 epithelium of the tubuli is adapted to a true excreting office, though 

 the walls of the tubuli must likewise excrete a little water. It is im- 

 possible to deny, moreover, that the glomeruli have also a true excret- 

 ing office. It is now believed to be certain that all the blood of the 

 renal arteries goes through these Malpighian corpuscles before it 

 reaches the tubules, and if it then becomes inspissated, the blood 

 which circulates around the tubules may be compared to the portal 

 blood of the liver, being, as it were, venous blood highly charged with 

 materials destined to be separated from it. In support of the view 

 that the Malpighian corpuscles separate the water, it has been urged 

 that in Birds and Reptiles, in which the urine is partly, or almost en- 



