570 STRUCTURE OF VERTEBRATA 



muscle ; a third acts on the melanophores of Amphibia, 

 causing expansion, whereas adrenahn provokes contraction. 



Thyroid glands . — These arise from the ventral wall of the 

 pharynx, as has been said. The active principle, thy- 

 roxin, is of known composition and has been synthesised. 

 It appears to act as a general stimulant to metabolism, all 

 over the body. A deficiency of thyroxin in adult life 

 leads to sluggishness, with a deposition of spongy tissue 

 under the skin (myxoedema) ; deficiency during growth 

 leads to tragic physical and mental under-development 

 (cretinism) ; it is often associated with an enlargement of 

 the inactive glands (goitre). When the active glands 

 enlarge, with excessive production of thyroxin (exophthal- 

 mic goitre, Graves' disease), excitability, emaciation, and 

 more serious symptoms may result. Often associated with 

 the thyroids are minute parathyroids^ which appear to form 

 an internal secretion regulating the metabolism of calcium 

 in the body ; death follows rapidly after their removal. 



Islets of Langerhans. — ^These are little clumps of tissue 

 embedded in the pancreas, the greater part of which is 

 taken up with the elaboration of digestive juices. The 

 islets form insulin , which controls sugar metabolism in the 

 body ; in diabetes mellitus there is a deficiency of insulin, 

 and the body loses its power to oxidise carbohydrates. 

 In some Teleosts the islet tissue is partly segregated and 

 distinct from the rest of the pancreas. 



Secretin, formed in the wall of the intestine, stimulates 

 the pancreas to secrete its digestive juice. The gonads 

 elaborate a complex of internal secretions, controlling 

 the appearance of the secondary sexual characters when 

 maturity is reached, and, in the female at least, co-ordinat- 

 ing the activity of the various organs taking part in the 

 sexual cycles. Other internal secretions have been supposed 

 to control gastric secretion, the activity of the heart, etc. 



Reproductive system. — The ovaries and testes are 

 developed from a ridge formed by a part of the epithelium 

 lining the abdominal cavity, this ridge constituting the 

 so-called germinal epithelium. 



In the male the proliferating germinal epithelium is 

 divided by embryonic connective tissue into numerous 

 follicles. The cells of the follicles form seminal mother-cells. 



