STRUCTURE OF THE VERTEBRATES 319 



premature senility is always accompanied by degenerated cor- 

 tical tissue. 



The functions of the medullary portion of the adrenals is 

 better understood. Adrenalin, the active substance of its secre- 

 tion, has been isolated and widely used. The effects are upon 

 the activity of the animal rather than upon growth and bodily 

 form, the hormone activating the muscles and autonomic nerv- 

 ous systems. 



8. Sex glands. The testis and ovary, like the pancreas, are 

 both duct and ductless in their structure and function. The 

 primitive function of the gonads is the production of reproduc- 

 tive cells; but in the land vertebrates (and probably in the 

 more primitive vertebrates) certain cells take on an endocrine 

 function. This function in the vertebrates is usually assigned to 

 the interstitial tissue which lies between the tubules of the 

 testis and the follicles of the ovary. In the latter organ, glandu- 

 lar bodies are developed after ovulation (corpora lutea) which 

 secrete a hormone of great physiological importance. It influ- 

 ences the fixation of the fertilized ovum on the uterine wall, and 

 the retention of the embryo in the uterus. In the rodents, at 

 least, the degeneration of the corpora lutea is largely responsi- 

 ble for the termination of pregnancy; and their proper function 

 helps initiate function in the mammary glands. 



Knowledge of the anatomical effects of the gonads has been 

 gained through castration and grafting experiments. The effects 

 of gonad removal are more prominent in the male, the individual 

 remaining in an undifferentiated stage with some female char- 

 acteristics. If castration occurs in youth it affects the shape of 

 the male pelvis, thorax and larynx. 



Removal of the sex glands of either sex during early youth 

 causes a delayed fusion of the epiphyses of the long bones with 

 the shaft. As a result a cartilage growth area remains after the 

 usual time of ankylosis. This permits continued growth of the 

 long bones, and such individuals have unusually long appendages. 

 Congenital, or very early, castration may result in apparent 

 gigantism, but the height is in the limbs, not in the body. 



Inter-relationships of the glands. It is always difficult to prove 

 a specific, independent action for a gland secretion. This is 

 particularly true for the hormones concerned with bodily growth 



