SECRETION. 333 



the gland-cells from the blood, or to speak more accurately, from the 

 plasma which exudes from the blood-vessels into the interstices of the 

 gland-textures. 



The best evidence in favor of this view is : 1st. That cells and nuclei 

 are constituents of all glands, nowever diverse their outer forms and 

 other characters, and that they are in all glands placed on the surface or 

 in the cavity whence the secretion is poured. 2d. That many secre- 

 tions which are visible with the microscope may be seen in the gland- 

 cells before they are discharged. Thus, bile may be often discerned by 

 its yellow tinge in the cells of the liver; spermatozoids in the cells of the 

 tubules of the testicles; granules of uric acid in those of the kidneys (of 

 fish); fatty particles, like those of milk, in the cells of the mammary 

 gland. 



Secreting cells, like the cells or other elements of any other organ, 

 appear to develop, grow, and attain their individual perfection by appro- 

 priating nutriment from the fluid exuded by adjacent blood-vessels and 

 elaborating it, so that it shall form part of their substance. In this per- 

 fected state, the cells subsist for some brief time, and when that period 

 is over they appear to dissolve, wholly or in part, and yield their con- 

 tents to the peculiar material of the secretion. And this appears to be 

 the case in every part of the gland that contains the appropriate gland- 

 cells ; therefore not in the extremities of the ducts or in the acini alone, 

 but in great part of their length. 



We have described elsewhere the changes which have been noticed 

 from actual experiment in the cells of the salivary glands, pancreas, and 

 peptic gland. 



Discharge of Secretions from glands may either take place as soon 

 as they are formed; or the secretion may be long retained within the 

 glands or its ducts. The former is the case with the sweat glands. But 

 the secretions of those glands whose activity of function is only occasional 

 are usually retained in the cells in an undeveloped form during the 

 periods of the gland's inaction. And there are glands which are like 

 both these classes, such as the lachrymal, which constantly secrete small 

 portions of fluid, and on occasions of greater excitement discharge it 

 more abundantly. 



When discharged into the ducts, the further course of secretions is 

 effected (1) partly by the pressure from behind; the fresh quantities of 

 secretion propelling those that were formed before. In the larger ducts, 

 its propulsion is (2) assisted by the contraction of their walls. All the 

 larger ducts, such as the ureter and common bile-duct, possess in their 

 coats plain muscular fibres; they contract when irritated, and sometimes 

 manifest peristaltic movements. Ehythmic contractions in the pancre- 

 atic and bile-ducts have been observed, and also in the ureters and vasa 

 deferentia. It is probable that the contractile power extends along the 



