HISTOLOGY 39 



sents a greyish appearance. The medulla is not entirely sur- 

 rounded by cortical tissue in all mammals. In some, the 

 medullary tissue clusters about the adrenal vein with part of 

 it surrounded by cortical tissue while the remainder is directly 

 under the adrenal capsule. Such a condition, for example, is 

 encountered in certain marsupials (Metachirus oppossum), in 

 the flying fox (Pteropus medius), and in the stone-marten 

 (Mustela foina) . 



The separation of the cortex and medulla is also very vari- 

 able. In the Monotremes, as in the spiny ant-eater (Echidna 

 aculeata) and in the duckbill (Ornithorhynchus paradoxus), the 

 medulla and cortex are most imperfectly divided, the former 

 penetrating far into the cortical tissue. On the other hand, 

 in some species, a definite band of connective tissue separates 

 the medulla from the cortex, at least during a certain period 

 of life. This is true of the mouse (Figure 9), little ant-eater 

 (Tamandua tetradactylus) , porcupine (Hystrix cristata), ele- 

 phant (Elephas indicus), armadillo (Dasypus septemcinctus) , 

 and man. The significance of this band of connective tissue 

 shall be discussed in the next chapter. 



There is considerable variation in the relative sizes of the 

 cortex and medulla of different mammals. In man and the 

 anthrapoid apes, the medulla is represented at birth by a 

 small strip of undifferentiated tissue. The medulla in these 

 animals is a post-natal development. The medulla forms a 

 relatively large proportion of the gland in the porpoise (Pho- 

 caena communis), rhinoceros (Rhinoceros unicornis), flying fox 

 (Pteropus medius), chimpanzee, and spider monkey (Ateles). 

 On the other hand, the medulla is relatively small in the guinea 

 pig (Cavia cobaya) and porcupine (Hystrix cristata). 



The adrenals of man and the higher apes Cgorilla, orangutan) 

 are characterized by folds or lobulations in the cortex which 

 are reflected in the medulla thus making the point of contact 

 of medullary and cortical tissue much more extensive than it 

 is in the more simply shaped adrenals of other species. The 



