CH. XXXVI.] FUNCTIONS OF THE KIDNEYS 543 



Into the urethra open a number of oblique recesses or lacunce, a 

 number of small mucous glands (glands of Littre), two compound 

 racemose glands (Cowper's glands), the glands of the prostate, and 

 the vas deferens. The prostate, which surrounds the commencement 

 of the male urethra, is a muscular and glandular mass. Its glands 

 are tubular and lined by columnar epithelium. 



* 

 THE FUNCTIONS OF THE KIDNEYS. 



The main function of the kidneys is to separate the urine from 

 the blood. The true secreting part of the kidney is the glandular i 

 epithelium that lines the convoluted portions of the tubules ; there * 

 is in addition to this what is usually termed the filtering apparatus : 

 we have already seen that the tufts of capillary blood-vessels called 

 the Malpighian glomeruli are supplied with afferent vessels from the 

 renal artery; the efferent vessels that leave these have a smaller 

 calibre, and thus there is high pressure in the Malpighian capillaries. 

 Certain constituents of the blood, especially water and salts, pass 

 through the thin walls of these vessels into the surrounding Bowman's 

 capsule which forms the commencement of each renal tubule. Though 

 the process which occurs here is generally spoken of as a filtration, 

 yet it is no purely mechanical process, but the cells exercise a selective 

 influence, and prevent the albuminous constituents of the blood from 

 escaping. During the passage of the water which leaves the blood j 

 at the glomerulus through the rest of the renal tubule, it gains the 

 constituents urea, urates, etc., which are poured into it by the secreting ' 

 cells of the convoluted tubules. 



The term excretion is better than secretion as applied to the kidney, 

 for the constituents of the urine are not actually formed in the kidney 

 itself (as, for instance, the bile is formed in the liver), but they are 

 formed elsewhere; the kidney is simply the place where they are 

 picked out from the blood and eliminated from the body. 



The Nerves of the Kidney. 



Nerves. The nerves of the kidney are derived from the renal 

 plexus of each side. This consists of both medullated and non- 

 medullated nerve-fibres, the former of varying size, and of nerve-cells. 

 Fibres from the anterior roots of the eleventh, twelfth, and thirteenth 

 dorsal nerves (in the dog) pass into this plexus. They are both \ 

 vaso-constrictor and vaso-dilator in function. The nerve-cells on 

 the course of the constrictor fibres are situated in the coeliac, mesen- 

 teric, and renal ganglia ; the cells on the course of the dilator fibres 

 are placed in the solar plexus and renal ganglia. 



These nerves are thus vaso-motor in function ; we have at present 



