ccxx SECRETING GLANDS. 



fluid in the animal body is effected without the necessary aid of any more 

 complicated apparatus. Thus, the exhalation of carbonic acid and watery 

 vapour from the interior of the lungs and air-passages, is probably produced 

 in this simple manner, although the structure of the exhaling membrane 

 is, for other reasons, complex ; and the discharge of fluid into cavities lined 

 by serous membranes, which is known to be preternaturally increased by 

 artificial or morbid obstruction in the veins, may be a case of the same 

 kiud. 



But another element is almost always introduced into the secreting struc- 

 ture, and plays an important part in the secretory process ; this is the nucleated 

 cell. A series of these cells, which are usually of a spheroidal or polyhedral 

 figure, is spread over the secreting surface, in form of an epithelium, which 

 rests on a simple membrane, named the basement-membrane, or meni- 

 brana propria. This membrane, itself extra vascular, limits and defines the 

 vascular secreting surface ; it supports and connects the cells by one of its 

 surfaces, whilst the other is in contact with the blood-vessels, and it may 

 very possibly, also, minister, in a certain degree, to the process of secretion, 

 by allowing some constituents of the blood to pass through it more readily 

 than others. But the cells are the great agents in selecting and preparing 

 the special ingredients of the secretions. They attract and imbibe into their 

 interior those substances which, already existing in the blood, require merely 

 to be segregated from the common store and concentrated in the secretion, 

 and they, in certain cases, convert the matters which they have selected into 

 new chemical compounds, or lead them to assume organic structure. A cell thus 

 charged with its selected or converted contents yields them up to be poured 

 out with the rest of the secretion, the contained substance escaping from it 

 either by exudation or, as is probably more common, by dehiscence of the cell- 

 wall, which, of course, involves the destruction of the cell itself. Cells 

 filled with secreted matter may also be detached, and carried out entire with 

 the fluid part of the secretion ; and, in all cases, new cells speedily take the 

 place of those which have served their office. The fluid effused from the 

 blood-vessels, no doubt, supplies matter for the nutrition of the secreting 

 structure, besides affording the materials of the secretion, the residue, when 

 there is any, being absorbed. 



Examples, illustrative of the secreting agency of cells, are afforded both 

 by plants and animals. Thus, cells, are found in the liver of various 

 animals, and especially of crustaceans and mollusks, some of which con- 

 tain a substance resembling coloured biliary matter, and others particles 

 of fat. Also, in the urinary organ of mollusks, cells are seen which 

 inclose little opaque masses of uric acid. The secretion of the sebace- 

 ous follicles in man often contains detached cells filled with fat ; and, 

 according to Mr. Goodsir's observation, the ink-bag of the cuttle-fish is 

 lined with an epithelium, the constituent cells of which are charged with 

 pigment, similar to that which imparts the dark colour to the inky secre- 

 tion. This last instance, as well as the production of spermatozoa, is an 

 example of the formation of new products within secreting cells, a pro- 

 cess further illustrated in plants, which afford abundant and decided 

 evidence of the production of young cells, spermatic filaments, starch- 

 granules, oil, various colouring matters, and other new compounds, in the 

 interior of cells. 



Both in animals and plants, the individual cells which are associated 

 together on the same secreting surface may differ from each other in the 

 nature of their contents. Thus, in the liver of mollusca some cells con- 



