294 ZOOLOGY. 



lated, not by afferent nerve-impulses, but by the carbon 

 dioxide contained in the blood. When the proportion oi 

 this gas in the blood increases, as during muscular exertion 

 or partial suffocation, the respiration centre is excited 

 and the respiratory movements increased. If the tension 

 of carbon dioxide in the blood is reduced, as by rapid 

 breathing for a short time or by an excess of oxygen in 

 the air breathed, the respiratory centre is not stimulated 

 and respiration ceases for a time, and we have the con- 

 dition called apnoaa. The carbon dioxide produced in the 

 body is a hormone which stimulates the respiratory centre. 



Starling and Bayliss have shown that the secretory 

 activity of the pancreas is due to a substance called secre- 

 tin, which is formed in the mucous membrane of the 

 duodenum by the action of the acid chyme, and which is 

 carried to the pancreas by the blood. 



The thyroid gland affords another example. Although 

 this is not a gland in the ordinary sense, with a duct 

 through which its secretion is delivered, it secretes specific 

 substances which when absorbed into the blood are neces- 

 sary for the normal development of the organs of the 

 young animal or child. Cretins are children whose de- 

 velopment is arrested in consequence of disease of the 

 thyroid gland : administration of the thyroid of calves or 

 of thyroid extract causes renewed growth and development 

 so that the child grows taller and more intelligent. 



The adrenal bodies produce a substance, adrenalin, which 

 has the same effect on the involuntary muscles as stipula- 

 tion of the sympathetic nerves. 



19. The Hormone Theory of Heredity. These facts, 

 especially those concerning the growth of special parts of 

 the body such as secondary sexual characters, or the whole 

 development as in the case of the thyroid gland, give as 

 before stated a new conception of the process of develop- 

 ment, and dispose finally of the contention that no process 

 is known to physiology by which modifications of the 

 development of the body could affect the properties of the 

 ovum. 



Darwin, in his theory of Pangenesis, suggested that all 



