PHYSIOLOGICAL 299 



to activity, hastening the development of the follicles and evoking 

 a flood of the ovarian hormones. 



The posterior lobe of the pituitary body is of cerebral origin, 

 and although its minute structure is like degenerate nervous tissue 

 there is a production of several hormones of great physiological 

 potency. One of these causes the capillaries to contract and the 

 blood- pressure to rise, and is thus naturally associated with increased 

 renal activity. But the intricacy of these inter-relations is suggested 

 by the fact that there seems to bean associated pituitary substance 

 that lowers blood-pressure. It may be, however, that this substance 

 is not peculiar to the pituitary body, but is common to many 

 organs. Yet another complication is indicated by the fact that 

 injury to the posterior lobe acts on the kidney and leads to diabetes 

 insipidus, which can be relieved by pituitary injection. Another 

 pituitary hormone has a general effect in exciting smooth or plain 

 muscle to contraction, and is so potent that a solution of one part 

 in a hundred thousand millions will work! Remarkable also is the 

 influence that changes in pituitary secretion exert on the con- 

 traction and expansion of the dark pigment-cells (melanophores) 

 in the frog's skin. 



The active substances produced by the pituitary body have not 

 yet been isolated, and there is no hint that they are represented 

 outside the sub-kingdom of Backboned Animals. It may be noted 

 that Prof. Herring of St. Andrews showed long ago that material 

 from the anterior part of the pituitary body may pass into the 

 cavity of the 'tween-brain, and thus into the cerebro-spinal fluid. 

 At the same time, apart from the regulation of bone-growth, and 

 perhaps the maintenance of capillary tone, it is difficult to say 

 what everyday value should be put to the credit of the powerful 

 secretions made by the pituitary body. For such influences as 

 those mentioned on kidney activity and milk yield are demonstrated 

 in experimental conditions, and we cannot at present say that 

 pituitary secretion is concerned with kidneys, mammary glands, 

 blood-pressure, and the like in ordinary normal life. It is therefore 

 very satisfactory to find Hogben's and Winton's brilliant demon- 

 stration that pituitary secretion is the main factor in co-ordinating 

 the everyday pigmentary responses that frogs and other Amphibians 

 make to the changing conditions of their environment. Others have 

 shared with these investigators in proving that the regulation of 

 the normal colour-response in these animals is mainly due to 

 fluctuating pituitary secretion. The mistake must not be made of 

 arguing from the frog's colour-change to that of other animals, 

 such as fishes (see section on Coloration). 



CORPUS LUTEUM. — The egg-cells or ova of mammals appear 

 within nests of cells in the ovary, called Graafian follicles, dis- 



