AND BLOOD PRESSURE 153 



the mammalian kidney will show that there can be no question of 

 a capillary pressure which can produce filtration therein. 



The formation of glands in the embryo displays the same pro- 

 gressive evolution from the simple to the complex state, as is 

 observed in ascending the animal scale. The most perfect and 

 complex glands of the higher animals resemble in embryo the 

 secretory organs of the lower animals. The arborescent ramifica- 

 tions of the blood vessels accompany the ducts in their develop- 

 ment, and in proportion as the development of a secreting plane 

 surface into a caecum and ramified caeca proceeds, the vascular 

 layer of the originally simple membrane spreads as a closely in- 

 vesting network around them. The ramified secreting tubes, 

 which, when the structure is simple as in Insecta and Crustacea 

 and in the pancreas of the rabbit, lie free freely and unconnectedly, 

 in proportion as their evolution is carried further acquire a common 

 covering or capsule ; and thus a solid organ is produced. The 

 vascular conditions in the simple and the complex are the same. 

 No one would be rash enough to suggest that filtration of fluid 

 could occur from the capillaries of the rabbit's pancreas through 

 the ramified tubules exposed m the mesentery in a thin sheet. 

 The tubules and the capillaries here are obviously at one and the 

 same pressure, that of the abdominal cavity, alike squeezed, by 

 the respiratory muscles, and pulsed by the wave of blood which 

 distends the abdominal arteries at each cardiac systole. There 

 is barely a positive pressure in the capillaries, but this, aided by 

 the rhythmic squeeze of the respiratory muscles, is sufficient to 

 maintain the onward flow of blood. The gland cells when excited 

 at times secrete their juice at a positive pressure, which with the 

 help of the peristaltic wave of the muscular wall of the tubules 

 drives the fluid into the intestine. Similarly in the case of the 

 kidney, the blood in the capillary networks, the tissue lymph, and 

 the urine in the tubules are all at one and the same pressure 

 the capillary-venous pressure. The whole kidney is expanded by 

 each arterial pulse, and drops of urine may be squeezed thereby 

 into the pelvis from the mouths of the tubules. The whole kidney 

 is rhythmically squeezed by the respiratory muscles. The tubules 

 are formed of watery cytoplasm, surrounded by lymph spaces full 

 of fluid, and networks of capillaries full of blood. There is nothing 

 of a rigid structure here, nothing of the nature of membranes which 

 can separate fluid at one pressure in one system of tubes from 



