4 o6 SECRETION. 



development of the cells, and their continuance in the 

 perfect state : and the secretions are discharged as the 

 constituent gland-cells degenerate and are set free. The 

 processes of nutrition and secretion are similar, also, in 

 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 tissues, 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 

 explanation of secretion as a process similar to nutrition ; 

 an explanation with which all the facts of the case are 

 reconcilable. 



It may be observed that the diversities presented by the 

 other constituents of glands afford no explanation of the 

 differences or peculiarities of their several products. There 

 are many differences in the arrangements of the blood- 

 vessels 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 in- 

 fluence the rate of the process and the quantity of the 

 material secreted. Cateris paribus, the greater the vascu- 

 larity 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 



