URINE. 391 



lobular (between the cortex and medulla), pass towards the centre 

 of the gland into the pyramids. They consist of bunches of 

 straight arterioles, which lie between the straight and the looped 

 tubules. Corresponding with these straight arteries are minute 

 straight veins, which carry the blood back to the vessels at the 

 base of the pyramids. 



In the kidney, then, we have three sets of capillary vessels, 

 which differ in their position, the form of their meshes, and their 

 relation to their parent artery. Probably the pressure exerted 

 by the blood in them, and the rapidity of its flow through them, 

 differ also : 



1. The capillaries in the glomeruli are loops collected into a 

 tuft by their covering of delicate epithelium. On account of 

 their relation to the afferent artery which ends abruptly in these 

 capillaries, and to the smaller efferent vessel that leads to a sec- 

 ondary plexus of capillaries, the pressure within the glomerulus 

 must be very great compared with that of the general capillaries 

 of the body, and must vary much with changes in local blood- 

 pressure. 



2. The secondary capillary plexus, with its narrow mesh work 

 closely investing the tubules, can only be under comparatively 

 trifling pressure which varies but little, on account of the blood 

 having first to pass through the capillaries of the glomerulus. 

 Their current of blood must also move slowly, since the bed of 

 the stream is here very great. 



3. The straight vessels, with long-meshed capillaries, in the 

 pyramids between the looped and straight tubules are unlike the 

 two preceding. In these straight vessels the blood probably flows 

 with greater velocity than in those around the convoluted tubes ; 

 and their blood-pressure is less than that in the glomeruli, but 

 greater than that in the intertubular capillaries. 



THE URINE. 



When freshly voided the urine of man in health is a clear 

 straw-colored fluid, with a peculiar aromatic odor. The intensity 

 of the color varies with the amount of solids the color being a 



