304 ZOOLOGY FOR MEDICAL STUDENTS chap. 



pharynx having been lost : the thyroid in its completed form is an 

 excellent example of what is known as a ductless gland. 



As already indicated the cells of the body are adapted to what may 

 be called an aquatic existence, to life in an internal watery medium — 

 very complex in composition owing to its containing in solution many 

 and varied products of the metabolism of the various tissues. It is 

 essential to the health of the body that this complexity should be 

 approximately normal, the various components being present in definite 

 proportions, although slight divergences from the normal may be harmless 

 and indeed may be of definite use in stimulating particular organs or 

 tissues to special activity. Thus an exceedingly slight increase in the 

 amount of carbon dioxide in the internal medium is at once followed 

 by increased activity in the respiratory movements so as to facilitate 

 respiratory exchange. Again there is produced in the metabolism of the 

 lining cells of the anterior portion of the intestine an obscure substance 

 to which the name secretin has been given : the presence of food in 

 the alimentary canal causes this to be produced in increased quantity : 

 and its presence in increased quantity in the internal medium, especially 

 in the circulating blood, at once brings on increased secretory activity 

 on the part of the pancreas — so that pancreatic juice is provided for the 

 digestion of the food.^ 



The primary function of a gland is the extraction of some specific 

 substance or substances from the circulating blood — i.e. from the internal 

 medium, and the passing it on, by way of the duct, to the exterior or to 

 some cavity in the body. It is, however, characteristic of the living 

 body that such action is always to a certain extent reciprocal, never 

 absolutely one-sided. In some cases the reciprocal action is very obvious, 

 as for example in the case of the respiratory exchange between a tissue-cell 

 and the blood. Oxygen passes from the blood to the tissue-cell and con- 

 versely carbon dioxide passes away from the tissue-cell into the blood. 

 In other cases the exchange between blood and cell is markedly unequal, 

 the process being very active in the one direction and comparatively 

 sluggish in the other. Such is the case in an ordinary gland where the 

 exchange from blood to gland-cell is conspicuous, giving rise to the 

 ordinary secretion, while the exchange from gland-cell to blood is com- 

 paratively inconspicuous and obscure, giving rise to what is called an 

 internal secretion. 



Although small in amount and obscure in nature these internal 

 secretions may be of high physiological importance : they frequently 



1 Such substances in the internal medium which bring about a, specific 

 reaction are commonly given the special name of hormones. 



