SECRETION. 189 



fibres to the gall-bladder run in the splanchuics and reach the liver through 

 the semilunar plexus. 



Normal Mechanism of the Bile-secretion. Bearing in mind the fact 

 that our knowledge of the secretion of bile is in many respects incomplete, 

 and that any theory of the act is therefore only provisional, we might picture 

 the processes concerned in the secretion and ejection of bile as follows : The 

 bile is steadily formed by the liver-cells and turned out into the bile-capil- 

 laries ; its quantity varies with the quantity and composition of the blood 

 flowing through the liver, but the formation of the secretion depends upon 

 the activities taking place in the liver-cells, and these activities are independ- 

 ent of direct nervous control. During the act of digestion the formation of 

 bile is increased, owing probably to a greater blood-flow through the organ 

 and to the generally increased metabolic activity of the liver-cells occasioned 

 by the inflow of the absorbed products of digestion. The bile after it gets 

 into the bile-ducts is moved onward partly by the accumulation of new bile 

 from behind, the secretory force of the cells, and partly by the contractions 

 of the walls of the bile- vessels. It is stored in the gall-bladder, and at inter- 

 vals during digestion is forced into the duodenum by a contraction of the 

 muscular walls of the bladder, the process being aided by the simultaneous 

 relaxation of a sphincter-like layer of muscle which normally occludes the 

 bile-duct at its opening into the intestine; both these last acts are under the 

 control of a nervous reflex mechanism. 



Effect of Complete Occlusion of the Bile-duct. It is an interesting 

 fact that when the flow of bile is completely prevented by ligation of the bile- 

 duct, the stagnant liquid is not reabsorbed by the blood directly, but by the 

 lymphatics of the liver. The bile-pigments and bile-acids in such cases may 

 be detected in the lymph as it flows from the thoracic duct. In this way they 

 get into the blood, producing a jaundiced condition. The way in which the 

 bile gets from the bile-ducts into the hepatic lymphatics is not definitely known, 

 but probably it is due to a rupture, caused by the increased pressure, at some 

 point in the course of the delicate bile-capillaries. 



KIDNEY. 



Histology. The kidney is a compound tubular gland. The constituent 

 uriniferous tubules composing it may be roughly separated into a secreting 

 part comprising the capsule, convoluted tubes, and loop of Henle, and a col- 

 lecting part, the so-called straight collecting-tube, the epithelium of which is 

 assumed not to have any secretory function. Within the secreting part the 

 epithelium diifers greatly in character in different regions; its peculiarities 

 may be referred to briefly here so far as they seem to have a physiological 

 bearing, although for a complete description reference must be made to some 

 work on Histology. 



The arrangement of the glandular epithelium in the capsule with reference 

 to the blood-vessels of the glomerulus is worthy of special attention. It will 

 be remembered that each Malpighiau corpuscle consists of two principal parts, 



