ix DUCTLESS GLANDS 305 



constitute contributions to the internal medium of the body which are 

 of the greatest moment to its healthy metabolism. Thus in the case of 

 the pancreas, in addition to the obvious secretion already alluded to, 

 there is believed to be produced some obscure contribution to the internal 

 medium the presence of which in normal proportion is essential to 

 healthy carbohydrate metabolism, its absence causing carbohydrate to 

 accumulate in the blood in the form of sugar. Now in the ductless glands 

 it is the production of these internal secretions which has become the 

 predominant function of the organ. In the case of the thyroid the 

 ordinary secretion is still produced but it no longer finds an exit into 

 the alimentary canal : the really important product is an obscure internal 

 secretion which passes from the gland into the internal medium by way 

 of the blood circulating through its capillaries. Abnormal proportion of 

 this internal secretion is known from investigations on the higher verte- 

 brates to cause serious disturbance of healthy metabolism, interfering 

 with mental and sexual development and with the growth of tissues, 

 particularly skeletal tissues. The disease known as " goitre " in human 

 beings is caused in this way : enlargement of the thyroid and interference 

 with its function being apparently brought on by infection by some 

 unknown microbe inhabiting drinking water in certain regions. And the 

 children born of a mother with goitre are apt to show the condition of 

 mental and physical deficiency known as " cretinism." 



Another ductless gland associated with the alimentary canal is the 

 thymus. This arises in the form of a series of outgrowths from the 

 epithelium lining the visceral clefts, close to their dorsal ends. The 

 several outgrowths fuse together, lose their connexion with the pharynx 

 and form an inconspicuous organ lying along the jugular vein. The 

 internal secretion produced by the thymus, whatever its nature may be, 

 would appear to be of special importance during the period of growth as 

 it commonly shrinks as a vertebrate approaches its adult size. 



There is evidence, as will appear later, that the vertebrates are 

 descended from ancestors which possessed a coelomic body-cavity divided 

 up into segmentally arranged compartments like those of an annelid 

 worm, each compartment formed by the fusion of an originally separate 

 right and left half. In the modern vertebrate, however, this segmenta- 

 tion disappears except as regards the dorsal portion of each compartment. 

 In this the cavity becomes obliterated, while the epithelial lining under- 

 goes an immense thickening and becomes converted into longitudinal 

 muscle-fibres each fibre running only the length of a single segment. 

 These segmentally arranged blocks of muscle-fibres known as myotomes 



x 



