2o6 ELEMENTS OF BIOLOGY 



cally controlled body functions. The outer portion of the gland, the 

 cortex, supplies a hormone that controls or at least has much to do 

 with the acidity-alkalinity level of the blood by reason of its effect 

 on the elimination of acid by the kidneys. 



The Thymus. The thymus, in mammals situated in the upper 

 thorax, has been commonly regarded as an endocrine gland, but 

 its function is not definitely known. In the human it disappears 

 with the approach of maturity.^ 



The Parathyroids. The parathyroids consist of a number of 

 small glands in the neck region. Although in Man their total mass 

 is quite small, they have a very important function and their de- 

 struction or removal results fatally. The hormone from these glands 

 in some way regulates the utilization of calcium by the body. Cal- 

 cium is an absolute essential to cell life, although the quantity uti- 

 lized is quite small except in the case of bone formation. 



The Gonads. The gonads of both sexes have a double func- 

 tion. In addition to the production of spermatozoa, the male gonad 

 or testis, secretes a hormone that appears to induce and to maintain 

 the secondary sex characters which make the body of the male ob- 

 servably different from that of the female. In Man the changes in 

 body form, voice changes, and other characters, both physiological 

 and psychical, that accompany maturity result from the effects of 

 this hormone on the metabolism of the several parts of the body 

 which undergo change. A section through the testis (Fig. 136) 

 shows that between the seminiferous tubules are masses of cells of 

 characteristic appearance, known as interstitial cells; these are 

 regarded as the portion of the gonad that is endocrine in function. 



In the female gonad or ovary (Fig. 137) are found similar masses 

 of interstitial tissue that secrete the female hormone. All the charac- 

 ters of the body that are distinctly female are regarded as due to the 

 influence of this hormone. In addition to the female hormone, cer- 

 tain temporary structures develop in the adult ovary that have to 



1 The pineal gland, dorsal to the brain stem, appears to be concerned with the delay of 

 sexual maturity. 



