14 INTERNAL SECRETION 



substances of physiological utility (the raw materials for which 

 are supplied by the circulating blood), by certain cells of a glandular 

 type ; the substances set free are not passed out on to a free surface, 

 but into the blood -stream. 



According to this definition, the products of ordinary meta- 

 bolism, and even the special products of metabolism arising 

 in such kinds of highly specialized tissues as muscle and nerve, 

 are excluded from the internal secretions. 



We have seen that externally secreting glands sometimes 

 manufacture and pour out substances which are waste products, 

 and are no longer of any use in the economy. These are 

 ''excretions." It is possible that some of the substances 

 elaborated by the internally secreting glands may also have 

 to be placed in the category of " excretions." They would 

 then be " internal excretions " (see p. 38). 



The terms " ductless gland " and " Blutgefassdriise " were 

 originally applied to a very varied group of structures, including 

 the thyroids and parathyroids, the adrenal bodies, the thymus 

 gland, the pituitary body, the spleen, and the lymphatic glands. 

 But some of these the spleen and the lymphatic glands 

 have not a "glandular" structure; that is to say, they do 

 not consist of epithelial " secreting " cells, and belong to quite 

 a different category of organs, namely, the " haemolymph " 

 series. The structures usually included at the present time 

 under the title of " ductless glands " are the thyroid gland ; 

 the parathyroid glands ; the adrenal body, consisting of 

 " cortex " and " medulla " ; chromaphil cells and bodies in 

 different regions ; the pituitary body, consisting of the " in- 

 fundibular " or " nervous " and the " glandular " portion ; 

 the thymus gland; and the corpus luteum. 1 The thymus 

 originates as an epithelial structure, but subsequently appears 

 to become largely converted into a lymphoid organ. 2 Its 

 morphological characters are therefore unique. 



It is believed that these " ductless glands " manufacture 

 and pour, directly or indirectly, into the blood-stream some 

 robfltanoe <>r -ubstances which are of service in the economy, 

 < -it her by supplying a need or by destroying other substances 

 which arc needless or positively harmful. This last function 



1 It 'm |.ossil,|.- that w. may have to add to the list the " Glandula insularis 



of N. Peii'l-'. 

 * See, however, discussion on p. 329. 



