THE SECRETION OF URINE. 411 



which frequently bears the name of Bowman, since he was the first to put it 

 forward, that certain constituents only of the urine are secreted after the fash- 

 ion of other secreting glands by the tubuli uriniferi, and that the rest of the 

 constituents, including a great deal of the water, with such highly soluble and 

 diffusible salts as pre-exist in adequate quantity in the blood, are, as it were, 

 filtered off by the glomeruli of the Malpighian capsules. We shall see, 

 later on, reason to doubt whether we are justified in applying the term 

 " filtration," which has a definite physical meaning, to the process by which 

 water and other substances pass from the bloodvessels of the glomerulus into 

 the lumen of the tubule ; for that process is, as we shall find, peculiar and 

 complex. But such a doubt need not prevent us from recognizing that the 

 whole act of secretion of urine consists of two parts, one of which is much 

 more closely dependent on the flow of blood through the kidney than is the 

 ordinary process of secretion, such as has hitherto come before us, and another 

 part which seems to bear the same relation to the flow of blood as does ordi- 

 nary secretion. 



That the work of the kidney is, to an unusual degree, dependent on the 

 flow of blood through it, seems suggested by the vascular arrangements ; for 

 these are extremely favorable to a full and rapid stream of blood through 

 the organ. The short and relatively broad renal artery comes off direct 

 from the abdominal aorta, where the blood-pressure is extremely high ; the 

 renal vein opens directly into the vena cava, where the blood-pressure is ex- 

 tremely low. Between the mouth of the renal artery and the mouth of the 

 renal vein the difference of pressure is very great, indeed ; and, as we have 

 seen in treating of the vascular system, it is the difference of pressure be- 

 tween two points of the vascular tract which is the actual cause of the flow 

 of blood from the one point to the other. The difference of pressure, in- 

 deed, which drives the blood through the limited area of the kidney is the 

 same difference of pressure which drives the blood along the abdominal 

 aorta down both legs back again to the vena cava. 



This free and abundant supply of blood is regulated, is either increased 

 or diminished, according to the needs of the moment, by the vasomotor sys- 

 tem ; this is shown by experimental and other results, which it will be profit- 

 able to study in some detail. Before entering into these details, however, it 

 will be well to call attention to the fact that when vasomotor events modify 

 the flow of blood through an organ, they produce their effects in one direc- 

 tion or another by working on arterial blood-pressure. Thus, as we shall see, 

 when stimulation or section of a nerve increases the flow of blood through the 

 kidney, it does so by increasing the pressure in the small vessels of the kid- 

 ney, including the capillary loops of the glomeruli. In such a case the walls 

 of the glomerular loops, through which the passage of materials to form 

 (part of) the urine takes place, are subjected to two influences on the one 

 hand, to a fuller, more rapid flow of blood past them, and, on the other, to 

 an increase of the pressure which that blood, as it passes along, exerts on 

 them. We shall have, subsequently, to discuss the share taken by these two 

 influences in determining and modifying the passage of material through 

 the walls of the glomerular loops ; and this will bear on the question of 

 filtration, to which we have above alluded ; but, for the present, it will be 

 convenient to deal with the effects of variation in blood-pressure apart from 

 this secondary question. 



346. The vasomotor mechanisms of the kidney. It may be shown ex- 

 perimentally that the kidney is supplied with a vasomotor mechanism as 

 well developed, perhaps, as that of any part of the body. By means of a 

 modification of the plethysmograph (Figs. 110, 111), we can readily observe 

 the variations which take place in the volume of the kidney. 



