DISCHARGE OF SECRETIONS. 323 



their obscurity : there is the same difficulty in saying why, 

 out of apparently the same materials, the cells of one gland 

 elaborate the components of bile, while those of another form 

 the components of milk, and of a third those of saliva, as there 

 is in determining why one tissue forms cartilage, another bone, 

 a third muscle, or any other tissue. In nutrition, also, as in 

 secretion, some elements of tissues, such as the gelatinous tis- 

 sues, are different in their chemical properties from any of the 

 constituents ready-formed in the blood. Of these differences, 

 also, no account can be rendered ; but, obscure as the cause of 

 these diversities may be, they are not objections to the ex- 

 planation of secretion as a process similar to nutrition ; an 

 explanation with which all the facts of the case are recon- 

 cilable. 



It may be observed that the diversities presented by the 

 other constituents of glands afford no explanation of the dif- 

 ferences or peculiarities of their several products. There are 

 many differences in the arrangements of the bloodvessels in 

 different glands and mucous membranes ; and, in accordance 

 with these, much diversity in the rapidity with which the 

 blood traverses them. But there is no reason for believing 

 that these things do more than influence the rate of the pro- 

 cess and the quantity of the material secreted. Cceteris pari- 

 bus, the greater the vascularity of a secreting organ, and the 

 larger the supply of blood traversing its vessels in a given 

 time, the larger is the amount of secretion ; but there is no 

 evidence that the quantity or mode of movement of the blood 

 can directly determine the quality of the secretion. 



The discharge of secretions from glands may take place as 

 soon as they are formed; or the secretion may be long re- 

 tained within the gland or its ducts. The secretion of glands 

 which are continually in active function for the purification 

 of the blood, such as the kidneys, are generally discharged 

 from the gland as rapidly as they are formed. But the secre- 

 tions of those whose activity of function is only occasional, such 

 as the testicle, are usually retained in the ducts during the 

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

 are like both these classes, such as the lachrymal and salivary, 

 which constantly secrete small portions of fluid, and on occa- 

 sions of greater excitement discharge it more abundantly. 



When discharged into the ducts, the further course of secre- 

 tions is effected partly by the pressure from behind ; the fresh 

 quantities of secretion propelling those that were formed before. 

 In the larger ducts, its propulsion is assisted by the contraction 

 of their walls. All the larger ducts, such as the ureter and 

 common bile-duct, possess in their coats organic muscular 



