304 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



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

 excelleni example of what is known as a ductless gland. 



Iready indicated the cells of the body are adapted to what may 



.died an aquatic existence, to life in an internal watery medium 



plcx in composition owing to its containing in solution many 



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



;:ial to the health of the body that this complexity should be 



approximati-ly 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 



ratory 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 



-lion of the food. 1 



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 



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



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



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



r 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- 



irbon dioxide passes away from the tissue-cell into the blood. 



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



the pnxos bring very active in the one direction and comparatively 



ish in tlu- other. Such is the case in an ordinary gland where the 



from Mood to gland-cell is conspicuous, giving rise to the 



onli: lion, while the exchange from gland-cell to blood is com- 



i\ely inconspicuous and obscure, giving rise to what is called an 



internal secretion. 



Although small in amount and obscure in nature these internal 

 may l>e of high physiological importance: they frequently 



the internal medium which bring about a specific 

 nonly given the special name of hormones. 



