HORMONES, DRUGS, AND TOXINS 719 



Mackenzie (1911) that the gland is not under the influence of the nervous system, 

 but that extracts of various organs, injected into the blood current of a cat in 

 lactation, cause secretion of milk. The organs found active were the pituitary 

 body, the corpus luteum, the pineal body, the involuting uterus, and the mammary 

 gland itself. The pituitary body is by far the most active; the substance 

 responsible is in the posterior lobe, and that of the bird is capable of exciting the 

 mammary gland of the cat (see Fig. 256). The foetus and placenta produce 

 hormones which inhibit the gland. Further analysis of the action of pituitary 

 extract was made by Hammond (1913). The effect is said not to be due to 

 pressing out of milk by contraction of muscle in the ducts, since, with other 

 evidence, after increase of secretion there is no sudden drop, followed by return 

 to the normal rate, as would be the case if the ducts had to be refilled. The daily 

 yield of goats was found to be only slightly increased by injections, so that 

 pituitary extract seems to act by setting free the constituents of the milk, rather 

 than by causing increased formation. The theory is suggested that the precursor 

 of milk-protein and lactose (perhaps a glyco-protein) is caused to take up water, 

 become hydrolysed, and by the increased osmotic pressure cause the inflow of 

 water to the cells and the washing out of the fat which has accumulated at the 

 ends of the cells. 



The Pituitary Body. In addition to the effect on the mammary gland just 

 described, and that on the kidney referred to on page 359 above, this organ has 

 other effects, especially on growth. 



The gland consists of two parts. From the posterior, nervous part the 

 hormones above mentioned are obtained, together with one which excites plain 

 muscle in general fo contraction. 



The anterior part secretes an eosinophile material, which, according to Herring 

 (1908), passes into the third ventricle, and thus into the cerebro-spinal fluid.. 



Disease of the gland shows it to have a powerful influence on growth and 

 metabolism. Gushing (1912) regards the state of acromegaly or gigantism as due 

 to excessive activity, and that of obesity, with eunuchoid changes, as due to 

 failure of pituitary hormone. The results of experimental interference are some- 

 what in dispute as yet. 



The Thyroid Gland. Here, again, analysis is difficult because of the double 

 nature of the organ. Definite information is wanting as to the relative functions 

 of the two parts. 



Absence of thyroid prevents growth, and produces the remarkable state of 

 myxoedema, associated with cretinism. Excess of the hormone causes Graves's 

 disease, exophthalmic goitre. In both cases curious nervous phenomena are 

 met with. 



Gaskell (1908, Chapter V.) brings strong evidence to show that the thyroid 

 gland of Ammoccetes, and therefore of vertebrates generally, is derived ancestrally 

 from the uterus of the original palseostracan. There is still a connection between 

 the generative organs and the thyroid which is a matter of popular knowledge, 

 and it seems not unlikely that remains of the internal secretion may have 

 continued when its original function ceased. 



The most interesting fact, chemically, with regard to the thyroid is the high 

 content in iodine, which appears to be present in a complex organic iodine 

 compound, united with a protein. That this is the active principle is shown by 

 the fact that the effect of thyroid substance, which is active even when taken by 

 the mouth, is in proportion to its iodine content. 



The Thymus. We are even more in the dark as to the function of this organ. 

 There is evidence that it has some function in normal growth. It is large in the 

 young animal, and becomes less and less as growth ceases. Its removal from the 

 young animal has been stated to cause retardation of growth. Hainan and 

 Marshall (1914), however, in careful experiments, found that removal of the 

 gland from young guinea-pigs had no effect on their growth, nor when the testes 

 were removed simultaneously. Castration, according to other investigators, leads 

 to hypertrophy of the thymus, but Hainan and Marshall could find no evidence 

 of any compensatory mechanism between the testes and the thymus. 



