SECRETION IN GENERAL. 34! 



the current toward the venous system. The alternate dilatation and compression of the 

 thoracic duct with the acts of respiration likewise aid the circulation, and they are more 

 efficient than any other force, except the vis a tergo. The action of the valves is pre- 

 cisely the same in the lymphatic as hi the venous system. 



CHAPTER XI. 



SECRETION. 



General considerations Differences between the secretions and fluids containing formed anatomical elements Classi- 

 fication of the secretions Mechanism of the production of the true secretions Mechanism of the production of 

 the excretions General structure of secreting organs Anatomical classification of glandular organs Classification 

 of the secreted fluids Secretions proper (permanent fluids; transitory fluids) Excretions Fluids containing 

 formed anatomical elements Physiological anatomy of the serous and synovial membranes Pericardial, peri- 

 toneal, and pleural secretions Synovial fluid Mucus Mucous membranes Mechanism of the secretion of mucus 

 Composition and varieties of mucus Microscopical characters of mucus General function of mucus Non- 

 absorption of certain soluble substances, particularly venoms, by mucous membranes Sebaceous fluids Physio- 

 logical 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 Function of the Meibo- 

 mian secretion Mammary secretion Physiological anatomy of the mammary glands Condition of the mam- 

 mary glands during the intervals of lactation Structure of the mammary glands during lactation Mechanism 

 of the secretion of milk Conditions which modify the lacteal secretion Quantity of milk General characters 

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

 trum Lacteal secretion in the newly-born. 



Secretion in General. 



THE phenomena classed by physiologists under the head of secretion are intimately 

 connected with the general process of nutrition. In the sense in which the term secre- 

 tion is usually received, it embraces most of the processes in which there is a separation 

 of material from the blood or a formation of a new fluid out of matters furnished by the 

 blood. The blood itself, the lymph, and the chyle, are no longer regarded as secre- 

 tions. These fluids, like the tissues, are permanent constituents of the organism, under- 

 going those changes only that are necessary to their proper regeneration. They are 

 likewise characterized by the presence of certain formed anatomical elements, which 

 themselves undergo the processes of molecular destruction and regeneration. These 

 characters are not possessed by the secretions. As a rule, the latter are homogeneous 

 fluids, without formed anatomical elements, except as accidental constituents, such as the 

 desquamated epithelium in mucus or in sebaceous matter. The secretions are not per- 

 manent, self-regenerating fluids, except when they perform simply a mechanical function, 

 as the humors of the eye, or the liquids in serous and synovial cavities. They are either 

 discharged from the body, when they are called excretions, or, after having performed 

 their proper function as secretions, are taken up again in a more or less modified form 

 by the blood. 



With the exception of those fluids which have a function almost entirely mechanical, 

 the relations of the secretions to nutrition are so close, that the production of many of 

 them forms almost a part of this' great function. It is difficult, for example, to con- 

 ceive of nutrition without the formation of the characteristic constituents of the urine, 

 the bile, and the perspiration ; and it is impossible, indeed, to study satisfactorily the 

 phenomena of nutrition without considering fully the various excrementitious principles, 

 such as urea, cholesterine, creatine, creatinine, etc., for the constant formation and dis- 

 charge of these principles by disassimilation create the necessity for the deposition of 

 new matter in nutrition. Again, the most important of the secretions, as contradistin- 

 guished from the excretions, are concerned in the preparation of food by digestion, for 

 the regeneration of the great nutritive fluid. 



