lo -March, 1909.] Elements of Ammal Physiology. 143 



THE THYROID GLAND.— This organ when micruscopically inves- 

 tigated has a structure distinctly gland-like except that its acini are 

 closed, that is, do not possess any duct. The acini are filled with a 

 glairy fluid which may be a true secretion, and if so must be absorbed 

 through the blood capillaries or by ihe lymph system. It is however a 

 matter of doubt whether the thyroid adds something to the blood or 

 whether it remo\es some poisonous product from the blood. The one thing 

 certain is that in the majority of mammals removal of the thyroid is fol- 

 lowed by muscular weakness, sluggish movement, and finally death. 

 In man loss of the thyroid produces the disease known as myxoedema. 

 The human child, bereft by any means of its thyroid, does not grow at 

 the normal rate and at the usual age of maturity may remain a dwarf 

 in body and a child in mind. The marvellous fact is that such a child 

 (a cretin as it is called) may be induced to grow normally by administer- 

 ing the thyroid gland of any mammal with its food. Similarly young 

 animals bereft bv surgical means of their thyroid will grow normally if 

 the thyroid of a kindred species is grafted in a suitable place in the body. 

 It has been suggested that the thyroid manufactures hormones which regu- 

 late growth but it must be admitted that thyroid extract administered to 

 an adult has the reverse action, namely, induces loss of weight. The 

 thyroid gland is placed in the neck caudal to the larynx. It is very 

 richly supplied with blood vessels. When greatly swollen it forms the 

 tumour known as goitre. Associated with it are small glands called the 

 para-thvroids the function of which is unknown. 



THE SUPRARENAL GLAND.— This organ has a double origin in 

 embryological development and displays a correspondingly twofold char- 

 acter in its tissue. The central portion, or medulla, arises from the 

 nervous system ; the outer portion consists of columns of epithelial 

 cells. Of the function of the outer portion we know nothing. As re- 

 gards our knowledge of the medulla we are in a more fa\'orable position. 

 It has now been placed beyond doubt that the medulla of the gland pro- 

 duces and adds to the blood a hurmone which can be obtained in crvs- 

 talline form and which is sold in the market under various names such 

 a? adrenalin, efinefhrin, hemiscitie, &c. The chemical constitution of 

 this substance has been determined ; it is a derivative of the well known 

 organic compound pyrocatechin. An extract of the suprarenal medulla, 

 or adrenali.n itself, if injected will produce all the effects of stimulation 

 of the thoracic autonomic or sympathetic ner\'es. Thus the arterioles 

 constrict, the pupil is dilated, the uterus contracts, the heart is accelerated, 

 the movem.ents of the alimentar\- canal are stopped, the hair is erected, 

 &c. The intensity of the effect produced bv even minute doses of 

 adrenalin is surprising. Thus tttVtt grain injected into the blood 

 stream of a dog may double the blood pressure owing to the powerful 

 constriction of the arterioles as well as the local stimulating action on 

 the heart. A dilute solution placed on a mucous membrane will so 

 constrict the vessels that complete pallor results. The effects of such 

 an injection or application are. howexer. verv temporary. If the 

 suprarenal glands be removed all those functions that depend upon 

 thoracic autonomic stimuli fall into abeyance and there is also a marked 

 muscular weakness and the animal invariably dies (Addison "s disease in 

 the human being). Adrenalin is largely used in medical and surgical 

 science on account of its poweiful propc^rties. If it accompanies a 

 local anaesthetic such as cocaine, wheii this is injected suljcutaneouslv or 

 intraspinally, it causes a much slower absorption of the drug. 

 If given by the mouth it produces constriction of the l)lood vessel.^ 



