306 SECRETION. 



Haller there is a full discussion of the influence of the diaphragm and of the 

 movements of the thorax upon the circulation of chyle. Colin always found 

 marked impulses in the flow of chyle from a fistula into the thoracic duct, 

 which were synchronous with the movements of respiration. With each act 

 of expiration the fluid was forcibly ejected, and with inspiration the flow 

 was very much diminished or even arrested. These impulses became much 

 more marked when respiration was interfered with and the efforts became 

 violent. The impulses were sometimes so decided, that the pulsations were 

 repeated in a long elastic tube attached to the canula for the purpose of col- 

 lecting the fluid. 



From all these considerations, it is evident that although there are many 

 conditions capable of modifying the currents in the lymphatic system, the 

 regular flow of the lymph and chyle depends chiefly upon the vis a tergo ; 

 but the vessels themselves sometimes undergo contraction, and they are sub- 

 ject to occasional compression from surrounding parts, which, from the exist- 

 ence of valves in the vessels, must favor the current toward the venous sys- 

 tem. The alternate dilatation and compression of the thoracic duct with 

 the acts of respiration likewise aid the circulation, and they are more effi- 

 cient than any other force, except the vis a tergo. The action of the valves 

 is precisely the same in the lymphatic as in the venous system. 



CHAPTER XL 



SECRETION. 



Classification of the secretions Mechanism of the production of the true secretions Mechanism of the 

 production of the excretions Influence of the composition and pressure of the blood on secretion- 

 Influence of the nervous system on secretion Anatomical classification of glandular organs Classifi- 

 cation of the secreted fluids Synovial membranes and synovia Mucous membranes and mucus- 

 Physiological anatomy of the sebaceous, ceruminous and Meibomian glands Ordinary sebaceous matter 

 Smegma of the prepuce and of the labia minora Vernix caseosa Cerumen Meibomian secretion- 

 Mammary secretion Physiological anatomy of the mammary glands Mechanism of the secretion of 

 milk Conditions which modify the lacteal secretion Quantity of milk Properties and composition of 

 milk Microscopical characters of milk Composition of milk Variations in the composition of milk- 

 Colostrum Lacteal secretion in the newly-born Secretory nerve-centres. 



THE processes of secretion are intimately connected with general nutri- 

 tion. In the sense in which the term secretion is usually received, it em- 

 braces most of the processes in which there is a separation of matters from 

 the blood by glandular organs or a formation of a new fluid out of materials 

 furnished by the blood. The blood itself, the lymph and the chyle, are in 

 no sense to be regarded as secretions. These fluids, like the tissues, are per- 

 manent parts of the organism, undergoing those changes only that are neces- 

 sary to their proper regeneration. They are likewise characterized by the 

 presence of certain formed anatomical elements, which themselves undergo 

 processes of molecular destruction and regeneration. These characters are 



