156 PRINCIPLES OF ANIMAL BIOLOGY 



8- or 9-foot stature with disproportionately long limbs. Too little of it 

 produces midgets, with disproportionately short limbs. The hormones 

 related to the sex organs and mammary glands are to be described in a 

 succeeding section. The hormones affecting the thyroid and adrenal 

 cortex have not been isolated ; but in an animal whose pituitary has been 

 removed these glands experience degenerative changes; and when 

 additional pituitary extract is injected, the thyroid and adrenal cortex 

 are enlarged. 



The posterior lobe of the pituitary produces at least two substances, 

 one of which stimulates contraction of the uterus in the reproductive 

 system, the other constricts the smaller arteries and so raises blood pres- 

 sure. Neither of these substances has been isolated. Injury to the 

 posterior lobe also deranges the uriniferous tubules of the kidneys, so 

 that they no longer resorb the great quantities of water from the filtrate 

 entering through Bowman's capsule. A large volume of dilute urine is 

 produced under these circumstances. 



The primary reproductive organs, ovaries and testes, produce hor- 

 mones which are responsible for the development of the secondary sex 

 characters, such as the beard and baritone voice in man, long tail feathers 

 in cocks, and the contrasted features of the females. They also govern 

 the mating behavior, and determine parental instincts. The principal 

 hormone in the male is testosterone (C19H30O2), isolated as a crystalline 

 compound. It is produced by the interstitial cells of the testis, not by the 

 germ cells nor the tubules which produce germ cells. The corresponding 

 hormone of the ovary (sometimes called estrogen though the name has 

 varied) is produced by the follicles, blastulalike spheres of cells surround- 

 ing the mature eggs. 



Other hormones may be produced by the pineal body above the 

 brain, which regulates the speed of sexual development, and the thyrnus 

 in the upper part of the chest, which is in some way related to sex develop- 

 ment and appears to control the production of the hard shell on birtl 

 eggs. Both of these organs are present in children, but the former degen- 

 erates into a fibrous structure and the latter disappears in youth. 



Reproductive Cycle. — The influence of the pituitary on other endo- 

 crine glands, mentioned al)Ove, hints at interrelations much more exten- 

 sive. Presumably not all the interrelations between the glands are 

 known, but one group of them has received considerable attention because 

 of its bearing upon medical practice. This is the group of glands and 

 other secreting structures which control the reproductive cycle in female 

 mammals. 



These females show a rhythmical change in their behavior, in that 

 periods of sexual excitement occur at regular intervals, separated by 

 periods of apathy. This rhythm of behavior depends on an alterna- 



