318 STRUCTURE OF THE VERTEBRATES 



bifurcation of the trachea. It is closely associated with the other 

 lymphoid tissues of the body, and in the mammal a hypertrophy 

 (overgrowth) of the thymus is correlated with an increased 

 amount of other lymph tissues. In the human its absolute weight 

 is greatest at about the time of puberty, although relatively it 

 is largest at birth. Removal of the gland has so far had little 

 effect upon the experimental animal, probably because of func- 

 tional as well as structural similarity between the thymus and 

 the lymph nodes. Clinical observations show that when the 

 thymus is hypertrophied general development proceeds slowly. 

 Feeding experiments on tadpoles (Gudernatsch) indicate the 

 correctness of the observations on the human. The effects are 

 the exact opposite of thyroid feeding, for thymus inhibits meta- 

 morphosis past the normal time of differentiation. 



6. Pancreas. The pancreas is a duct gland emptying into the 

 duodenum, with small "islands" of endocrine tissue scattered 

 throughout its mass. The extract of the endocrine portion (the 

 islands of Langerhans) is known as insulin. This hormone in- 

 fluences the absorption of carbohydrates by the cells, and thus 

 the general carbohydrate metabolism. Diabetes is the patho- 

 logical effect of improper function. 



7. Adrenal glands. The adrenal glands of the mammal lie 

 near the kidneys, and have a mesodermal cortex and an ecto- 

 dermal medulla. The functions of the two are entirely different, 

 and they may be considered as composites of two different 

 glands. In the fish the mesodermal and ectodermal glands are 

 separate. The ectodermal portion lies as two strips of chromaffin 

 tissue in close contact with the sympathetic system of ganglia. 

 The mesodermal glands are bands of inter-renal tissue parallel- 

 ling the mesonephros. In the reptiles the two portions of the 

 gland have come into close contact. In the mammals the ecto- 

 dermal gland is completely surrounded by the mesodermal por- 

 tion, forming a single structure. 



The function of the cortex has not been solved, although 

 extracts from it have been isolated which correct the effects of 

 Addison's disease. In the mammal the removal of the cortical 

 portion is followed by death. Clinical evidence indicates that 

 the glands are related to anatomical development. A peculiar 

 condition characterized by almost complete hairlessness and 



