396 SECRETING GLANDS. 



SECRETING GLANDS. 



The term gland has been applied to various objects, differing widely from each 

 other in nature and office, but the organs the structure of which it is proposed to 

 consider generally in the present chapter, are those devoted to the function of 

 secretion. 



The element which plays the most important part in the secretory process is the 

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

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

 generally rests on a membrane, named the basement-membrane, or membrana 

 propria. This membrane, itself extravascular, limits and defines the secreting sur- 

 face ; it supports and connects the secreting cells on one side, whilst on the other it , 

 is bathed with lymph and is in close proximity to the blood-vessels. The basement- 

 membrane when present is formed of flattened cells, which may be united edge to 

 edge to form a complete limiting membrane, or may be branched and united by their 

 processes with gaps between them (fig. 452). But a basement-membrane is not univer- 

 sally present, and it is the cells that are the chief agents in selecting and preparing 

 the special ingredients of the secretions. They take into their interior those sub- 

 stances which, already existing in the blood, require merely to be segregated from 



the common store and concentrated in the secre- 

 tion, and they, in certain cases, convert the matters 

 which they have selected into new chemical com- 

 pounds, 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 by bursting 

 and destruction of the cell itself. Cells filled with 

 Fig. 452.-MKMBRANA PROPRIA OP gecrete( i ma ttei' may also be detached, and carried 



TWO ALVEOLI OF THE ORBITAL J . . , 



GLAND OF THE DOG. (Heidenhain out entire with the fluid part of the secretion ; and, 

 and Lavdovsky.) in all cases, new cells speedily take the place of 



those which have served their office. The fluid 



effused from the blood-vessels supplies matter for the nutrition of the secreting 

 structure besides affording the materials of the secretion. 



Changes in the cells during activity. Since the materials for secretion 

 are selected or prepared by the cells, it is not surprising to find that the cells of a 

 secreting gland differ considerably in appearance according as the gland is in a con- 

 dition of rest or activity (figs. 453, 454). In the former case the materials for 

 secretion may have been accumulating within the cells and may be detected within 

 them, whereas in the latter case, if the secretion have been proceeding for some time, 

 the cells may be emptied of the accumulated material, and in many instances may 

 themselves be partially or entirely destroyed, owing to the disruption of their pro- 

 toplasm in the process of discharge of the secreted matter. In some glands however 

 the accumulation of the materials of secretion within the cells does not go on to any 

 great extent during rest, but begins with the increased activity of the gland con- 

 sequent on stimulation whether natural or artificial, and proceeds up to a certain 

 point, after which the process of discharge of the accumulated material begins. 

 But according to Heidenhain and Langley the processes of growth of the proto- 

 plasm, formation of material for secretion, and discharge from the cell may all go on 

 simultaneously, the material becoming formed by or from the cell protoplasm on the 

 one hand, and discharged on the other hand into the commencement of the duct. 



