22 



THE NEW CELL DOCTRINE 



ternal secretions the products of the thyroid gland, the hy- 

 pophysis and the suprarenal bodies. The thyroid gland in- 

 fluences the condition of the muscles; the hypophysis, the 

 growth of bones; and the suprarenal capsule the activity of 

 nerves. In passing it should be remarked that the phenom- 

 ena are not simple, but complicated. In all cases, however 

 we see that many cells of one kind depend as to their structure 



Fig. 14. — Section of a thyroid gland. The organ consists of closed cavities, 

 each of which is bordered by a layer of epithelial ceOs. Since the gland has no 

 duct, the secretion can be carried off only by the blood. — After Koelliker. 



and their activity upon the influence of these internal secre- 

 tions. It is not the case of a single cell, but always of many 

 which have the same constitution. 



The brilliant investigations of Ehrlich and others have 

 founded the new doctrine of immunity. In this case we 

 have to do with the phenomenon similar to that of in- 

 ternal secretion. An animal becomes poisoned by patho- 

 genic organisms, and then forms itself a contra-poison, or so- 



