Mirfa 



SIMPLE ISOLATED CELLS. SECRETING CELLS. 149 



surface of the membrane. The accompanying figure (27) represents 

 two follicles of the liver of the Common Crab, which are seen to be filled 

 with secreting cells ; it seems evident, from the comparative sizes of 

 these cells in different parts, that they originate at the blind extremity 

 of the follicle, where there is a germinal spot ; and that, as they recede 

 from that spot, they gradually increase in size, and become filled with 

 their characteristic secretion, being at the same time pushed onwards 

 towards the outlet by the continual new growth of cells at the germinal 

 spot. In Fig. 28 are shown the corresponding ultimate follicles of the 

 Mammary gland ; filled, like the preceding, with secreting cells. 



289. The whole of the acts, then, by which the separation of the dif- 

 ferent Secretions from the Circulating fluid is accomplished, really con- 

 sist in the growth and nutrition of a certain set of cells, usually covering 

 the free surfaces of the body, both internal and external, or lining cavi- 

 ties which have a ready communication with these by means of ducts or 

 canals.* These cells differ widely from one another, in regard to the 

 kind of matter which they appropriate and assemble in their cavities ; 

 although the nature of their walls is probably the same throughout. 

 Thus we find biliary matter and oil, easily recognisable by their colour 

 and refracting power, in the cells of the liver; milk in the cells of the 

 "lammary gland ; sebaceous or fatty matter in the cells of the sebaceous 



Hides of the skin ; and so on. All these substances are derived from 



e blood ; being either contained in it previously, or being elaborated 

 from its constituents by a simple process of transformation, as, for 

 example, that which converts the albumen of 'the blood into the caseine 

 of milk. Hence they may be considered as the peculiar aliments of the 

 several groups of cells ; whose acts of nutrition are the means of drawing 

 them off, or secreting them, from the general circulating fluid. When 

 they have attained their full growth, and accomplished their term of life, 

 their walls either burst or dissolve away, and thus the contents of the 

 cells are delivered into the cavity, or upon the surface, at which they 

 are required. Now as all the canals of the glands open either directly 

 outwards upon the surface, or into cavities which communicate with the 

 exterior, it is evident that the various products of the action of these 

 epithelial cells must be destined to be cast forth from the body. This 

 we shall find to be the case ; some of them, as the bile and urine, being 

 excretions, of which it is necessary to get rid by the most direct channel ; 

 whilst others, like the tears, the saliva, the gastric fluid, the milk, &c., 

 are separated from the blood, not so much for its purification, but because 

 they are required to answer certain purposes in the economy. 



240. Now whilst thus actively concerned in the Nutritive functions 

 of the economy, and exercising in the highest degree their powers of 

 selection and transformation, these Secreting cells appear to have nothing 

 to do with the operation of Reproduction. We have seen that they do 

 not even regenerate themselves ; all their energies being, as it were, con- 

 centrated upon their own growth ; and the successive production of new 

 broods of them being provided for by other means. Throughout the 

 organized creation, it appears to be necessary that the true act of Gene- 





* The Synovial secretion is perhaps the only one which is poured into a closed sac. 



