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 i*v 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 mig%t 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 differs 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 Malpighian corpuscle consists of two principal parts, 



