GROSS ANATOMY 19 



from the vena cava but in some species {e.g., the rabbit) it lies 

 in close contact to the large abdominal veins. 



GENERAL MORPHOLOGY 



The adrenals of the different mammalian species differ in 

 their gross anatomical appearance and topographical arrange- 

 ment as shown in Figure 2 in which are reproduced the photo- 

 graphs of representative types of glands. The apposition of 

 the glands to the kidneys or the vena cava gives depressions 

 which modify the simple bean-shaped or spherical forms 

 observed in some species. 



The weight of the adrenals compared to the body weight or 

 kidney weight in different animals has been the subject of 

 several investigations since Meckel 448 and Cuvier 144 first ini- 

 tiated this type of comparative study. In general the adrenals 

 of most mammals weigh from 0.01 to 0.02 per cent of the total 

 body weight. The guinea pig and several of its wild relations 

 are exceptional in this respect, their adrenals weighing almost 

 ten times as much per unit of body weight than other animal 

 species. The relatively large gland of the guinea pig is com- 

 posed chiefly of cortical tissue. 108 The medulla bears the same 

 relative relation to the body weight as it does in other 

 mammals. 183 



Except during the period of degeneration of the androgenic 

 zone, described in Chapter IV, the adrenals increase in size as 

 the body grows (although not proportionately) except in 

 fowls 183 where there is a marked reduction in adrenal size after 

 maturity despite an increase in body weight. This may sug- 

 gest the presence of androgenic tissue in the fowl's adrenal. 79 

 In the guinea-pig the medulla attains its full development by 

 the end of the first month of life, subsequent growth of the 

 gland being limited to the cortex. In other animals likewise 

 the medulla increases but slightly in size after the first period 

 of life. In birds the medulla forms a very large fraction of the 

 whole gland, the cords of medullary cells being interwoven in a 



Wl 



