154 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



converted into a substance of a more directly organizable character. 

 The special function of the red corpuscles peculiar to Vertebrated animals, 

 though not jet accurately known, seems intimately connected with the 

 process of Respiration. Next we have various groups of cells, external 

 to the vessels, on the free surfaces of the body ; whose office it is to draw 

 from the blood certain materials, which are destined for Secretion or 

 separation from it ; either for the sake of preserving that fluid in its re- 

 quisite purity, or for answering some other purpose in the system. 

 These cells grow at the expense of the substances which they draw into 

 themselves from the blood ; and on their dissolution, they cast forth their 

 contents on the free surfaces communicating with the exterior of the 

 body, to which they are in time conveyed. And, lastly, we have a 

 special set of Grenerative cells, destined in the one sex to prepare the 

 germs of new beings ; and in the other to elaborate a product essential 

 to their fertilization. 



247. The cells which are thus the active instruments of the Organic 

 functions, are usually produced and succeed one another with a rapidity 

 proportional to the energy of those functions, though the causes which 

 influence their growth and decay are not always evident. Thus it is 

 certain that, cceteris paribus, the rate of production of the Secreting 

 cells depends upon the abundance of the materials supplied by the circu- 

 lating current, which they are destined to eliminate from it. But this 

 is by no means the sole condition of their development ; for, as we shall 

 see hereafter, these materials may accumulate unduly in the blood, 

 through the insufficient activity of the cells which are destined to sepa- 

 rate them ; whilst, on the other hand, the presence of certain substances 

 in the blood appears to accelerate their production. Of these stimuli, 

 Mercury is one of the most powerful ; and we have continual opportu- 

 nities of witnessing its effects, in giving an increased activity to the 

 secreting actions. There is probably not a gland in the body, which is 

 not in some degree influenced by its presence in the blood ; but the liver, 

 the kidneys, the salivary glands, and the glandule of the intestinal 

 canal, appear to be those most affected by its stimulating powers. The 

 action of the glands, in other words the development of the secreting 

 cells, appears to be influenced by mental emotions ; being sometimes accele- 

 rated, and sometimes retarded, through their agency. This is especially 

 the case in regard to the secretion of Milk, Tears, Saliva, and Gastric juice. 

 It seems probable that the influence thus manifested is partly exerted 

 through the capillary circulation, which is known to be powerfully affected 

 by mental emotions, as in the acts of blushing and erection ; and that the 

 increased production of the secretion is immediately due to the increased 

 flow of blood to the gland. But there are other phenomena which show 

 that the development and actions of the secreting cells are more directly 

 influenced by the nervous system ; these will be hereafter considered 

 (CHAP, ix.) 



5. Of Cells connected together as permanent constituents of the Tissues. 



248. We now pass on to consider those Cells, which enter as compo- 

 nent elements into the solid and permanent fabric of the body, and which 



