394 OF SECRETION. 



We have seen that the retention of Carbonic acid in the blood for even 

 a few minutes is fatal, both by putting a stop to the circulation, and 

 by acting unfavourably upon the substance of some of the most impor- 

 tant organs in the body. The same fatal result attends the retention of 

 Urea and of Biliary matter, which are among the other products of the 

 decomposition of the tissues; but, although as certain, it is not so 

 speedy, because the general circulation is not affected by the loss of 

 secreting power on the part of the Kidneys and the Liver, and because 

 the accumulation of the noxious matter is slower. On the other hand, 

 the ingredients that are met with in those secreted fluids, which are 

 destined to answer some purpose in the economy, almost invariably 

 bear a close correspondence with the ordinary materials of the blood. 

 Thus in the Salivary, Gastric, Pancreatic, and Lachrymal fluids, the 

 principal part of the solid matter consists of the saline and of the albu- 

 minous constituents of the blood, the latter in a more or less altered 

 condition. In Milk, again, we trace the ordinary constituents of the 

 blood, with very little change. Thus it appears, that the separation of 

 these fluids is not required so much to maintain the Blood in a state of 

 purity, as to supply what is needed for some subsequent operation in 

 the economy ; and hence, if the secreting process be interrupted, in the 

 case of any one of them, the suspension has usually no further effect, 

 than that of disturbing the process to which the fluid is usually subser- 

 vient. If the secretion of Gastric fluid be checked, for example, under 

 the influence of strong mental emotion, the Digestive operation is pre- 

 vented from taking place. 



712. The essential character of the true Secreting operation seems 

 to consist, not in the nature of the action itself, for this is identical 

 with those of Assimilation and Nutrition, being, as we have seen ( 239), 

 a process of cell-growth, but in the position in which the cells are 

 developed, and the mode in which the products of their action are 

 afterwards disposed of. Thus the cells at the extremities of the intes- 

 tinal Villi ( 243), the cells of which the Adipose tissue is made up 

 ( 257), and the cells of which the greater part of the substance of the 

 Liver is formed (. 239), all have an attraction for fatty matter ; and 

 draw it from the neighbouring fluids, at the expense of which they are 

 developed, to store it up in their own cavities. But the cells of the 

 first kind, when they have come to maturity, set free their contents, 

 which are delivered to the absorbent vessels, to be introduced into the 

 circulating current ; those of the second kind seem more permanent 

 in their character, and retain their contents, so as to form part of the 

 ordinary tissues of the body, until they are required to give them up 

 for other purposes, when the matters which they have temporarily 

 separated from the circulating current, are restored to it again without 

 change ; and the cells of the third class, when they liberate their con- 

 tents (which they are continually doing), cast them forth into the 

 hepatic ducts, by which they are carried into the intestinal canal, 

 whence a portion of them at least is directly conveyed out of the body. 



713. It is, then, in the position of the Secreting cells, which causes 

 the product of their action to be delivered upon a, free surface, commu- 

 nicating, more or less directly, with an external outlet, that their 



