HORMONES 471 



output to rise. It then has the same effects as stimulation of 

 the sympathetic system ; these include a rise in blood pressure, 

 dilatation of the pupil, erection of hairs, and breakdown of 

 glycogen to lactic acid. 



A good example of a specific hormone is the secretin produced 

 by the intestinal mucosa when acid comes in contact with it. The 

 hormone is carried in the blood to the pancreas, which it stimu- 

 lates to produce bicarbonate ; the whole action is obviously 

 self-regulating. 



Hormones are also secreted by ordinary glands. Insulin, 

 shortage of which causes diabetes, is produced by certain cells 

 (islets of Langerhans) in the pancreas. The testes and ovaries 

 not only form spermatozoa and ova but also liberate hormones, 

 of which some influence the development of secondary sexual 

 characters in the growdng individual, and others, co-operating 

 wdth the pituitary, control the reproductive cycle. 



The stimulus that starts the cycle in the female is a seasonal 

 change in the outside world. In most mammals it is the increasing 

 intensity of light or length of day in spring, acting through the 

 eyes, and it probably causes the hypothalamus of the thalam- 

 encephalon to secrete a hormone which travels in the hypophyseal 

 portal system to the anterior pituitary. In other mammals, 

 such as sheep and cattle, the reverse light-changes have the 

 same effect, so that breeding takes place in autumn. The anterior 

 pituitary is stimulated to produce two hormones. The follicle- 

 stimulating hormone (FSH) causes the maturation of one or more 

 Graafian follicles and also the secretion of the hormone oestrogen 

 by the ovary. Oestrogen induces the phenomena of oestrus, and 

 also reduces the activity of the pituitary in secreting FSH, so 

 that no more follicles ripen. In many mammals the second or 

 luteinising hormone of the pituitary (LH) is present in large 

 enough quantities to cause ovulation when the follicle is ripe, 

 but in the rabbit and some others it is liberated in sulhcient 

 amount only after copulation. In all, it causes the transformation 

 of the folHcle into the corpus luteum. This produces another 

 hormone, progesterone, which prepares the uterus and vagina 

 for the attachment of the foetus and birth, causes the develop- 

 ment of the mammary glands, and inhibits oestrus and ovulation. 

 The maintenance of the corpora lutea is brought about by pro- 

 lactin, a third or luteotropic pituitary hormone (LTH), which 

 stimulates milk production also. 



