148 



DISCOVIiRY 



Some Recent Work on 

 the Ductless Glands 



By Lancelot T. Hogben, M.A., D.Sc. 



Itnjuridl CnUetfc of Science and Teclmolofjij 



It is a matter of common knowledge that the nervous 

 system directs into appropriate channels of response 

 the stimuli which an animal receives from its surround- 

 ings. But the nervous system does not constitute the 

 sole mechanism known to be instrumental in co- 

 ordinating the behaviour of the organism. During the 

 past twenty-five years it has been increasingly recog- 

 nised that, as Brown Sequard was among the first to 

 foresee, the blood stream provides an alternative 

 channel by which chemical compounds produced in 

 one part of the body may evoke a reaction in another 

 organ remotely situated with respect to it. As an 

 illustration a discovery made at the beginning of this 

 century by Bayliss and Starling will serve. When 

 food is presented to an animal like the dog, it secretes 

 saliva ; nervous impulses pass from the organs of 

 vision, taste, or smell to the brain, and are directed 

 thence into the motor nerves which stimulate the 

 salivary glands to activity. Similarly, when food 

 enters the small intestine, the digestive gland known 

 as the pancreas begins to secrete actively. But the 

 way in which this organ is activated is entirely different. 

 The acid food coming from the stomach acts upon a 

 substance present in the intestinal wall to set free a 

 compound called by Bayliss and Starling " secretin." 

 This secretin diffuses into the blood stream, and is 

 carried sooner or later to the pancreas, on which it has 

 a specifically' excitatory action. 



Such chemical messengers are spoken of as hormones. 

 And there exist in the body a number of organs whose 

 unique function is to regulate various activities by 

 setting free into the blood stream hormones or internal 

 secretions which act specifically either in producing 

 such immediately visible responses as muscular move- 

 ment or secretion in other organs or the more subtle 

 form of regulation involved in the growth processes. 

 Such structures are known as ductless or endocrine 

 glands. Two of the most important of the ductless 

 glands are the thyroid, situated on the ventral side of 

 the throat, and the pituitary, encased in a depression 

 of the skull at the base of the brain. Both glands are 

 of the utmost medical importance ; but they are also 

 significant to an understanding of some of the most 

 baffling problems of animal physiology ; and it is the 

 purpose of this article to outline some recent discoveries 

 relating to the function of the thyroid and pituitary 

 glands which open up wide fields for inquiry though 

 not of themselves directly utilitarian. 



The interest of biologists became focused on the 



(juestion of internal secretion in its non-medical 

 bearings, especially through a discovery made ten 

 years ago by Gudernatsch. This worker experimented 

 with dieting tadpoles of the common frog on various 

 kinds of tissue such as brain, liver, etc., including the 

 ductless glands such as the thyroid. He employed 

 an enormous variety of tissue foods, and he found that, 

 with the exception of those individuals which were 

 brought up on a thyroid diet, the tadpoles grew at 

 much the same rate and transformed into frogs at 

 about the same time as they do in natural surroundings. 

 The tadpoles reared on a thyroid diet were, however, 

 quite exceptional in transforming into frogs at a much 

 earlier date and long before attaining the dimensions 

 with which metamorphosis is ordinarilv associated. 

 How potent is the thyroid tissue in effecting the 

 change may be judged from the result of later experi- 

 ments by Swingle (191S) on the large American frog 

 Rana cateshiana. which, unlike our own species, takes 

 three seasons in the ordinary course of events to reach 

 the adult condition. Fed on fresh thyroid gland Rana 

 cateshiana will complete its larval life and transform 

 into a pygmy frog at the age of six weeks. 



Gudernatsch's discovery was soon confirmed by the 

 observations of several other workers — Morse, Barthele- 

 mez, and Swingle — who obtained corresponding results 

 with other larva of amphibians (toads, newts, sala- 

 manders, etc.). It does not in itself establish a relation 

 between the thyroid gland's activity and the transfor- 

 mation of the tadpole into the frog ; for the thyroids 

 used in the experiments were taken from sheep and 

 oxen — animals widely separated from the frog and its 

 allies. However, the facts which had been elicited 

 stimulated other workers to e.xplore the ground more 

 thoroughly. Bennet Allen, an American zoologist, 

 succeeded (1916-18) in overcoming the technical 

 difficulties of removing the thyroid from tadpoles of 

 the toad. The delicate operation of extirpating the 

 rudiment of the thyroid from very young larva was 

 carried out with a cataract needle under a microscope. 

 The wound produced heals within an hour, and fortu- 

 nately the amphibia are free from susceptibility to 

 septic poisoning. Tadpoles deprived of their thyroids 

 at this early stage grow in a perfectly normal manner, 

 until they reach the age at which metamorphosis 

 would be expected to occur. Instead of undergoing 

 transformation at this juncture into the adult form, 

 they retain their gills and tails, the forelimb rudiments 

 fail to break through the skin, and the animals con- 

 tinue to grow, attaining dimensions far exceeding 

 those of the ordinary tadpole. In short, they lack the 

 power to undergo metamorphosis under normal con- 

 ditions. Thyroidless tadpoles do nevertheless com- 

 plete their development and emerge from the larval 

 phase, if, as Swingle was able to show later (1918), they 



