FUNCTIONS OF THE DUCTLESS GLANDS. 461 



soon subsides into one of less activity, which merely serves to keep up its pro- 

 portion to the rest of the body ; and its increase usually ceases altogether at the 

 age of about two years. From that time, during a variable number of years, it 

 remains stationary in point of size ; but, if the individual be adequately nour- 

 ished, it gradually assumes the character of a mass of fat, by the development 

 of the corpuscles of its interior into fat-cells, which secrete adipose matter from 

 the blood. This change in its function is most remarkable in hybernating 

 Mammals ; in which the development of the organ continues, even in an in- 

 creasing ratio, until the animal reaches adult age, when it includes a large 

 quantity of fatty matter. The same is the case, generally speaking, among 

 Reptiles. 1 



487. The Thyroid body accords rather with the Supra-renal capsules, than 

 with the Thyrnus, in its elementary structure; for it consists of a number of 

 isolated vesicles, which do not communicate with any common reservoir. These 

 vary in diameter, in the Human subject, from l-2000th to l-85th of an inch; and 

 they contain an albuminoid plasma, 3 which is either faintly granular, or of a 

 somewhat oily aspect, amidst which are seen a number of corpuscles, of an 

 average diameter, of l-3000th of an inch, of which the greater part are in the 

 condition of nuclei, whilst some have advanced to that of cells. These corpus- 

 cles seem rather to occupy the position of an epithelium within the vesicles, 

 than to float freely in their contained fluid. The vascular supply of the Thyroid 

 body is extremely abundant ; and, as in the preceding instances, the subdivisions 

 of its arteries form a very minute capillary plexus upon the membrane of the 

 vesicles. The Lymphatics have not been traced far into its substance. The 

 development of the Thyroid body has been shown by Mr. Gray (loc. cit.) to 

 be closely accordant with that of the " ductless glands" already described. This 

 body originates in two separate masses of blastema, one at each side of the root 

 of the neck, close to the separation of the carotid and subclavian vessels, and 

 between the trachea and the branchial clefts, but quite independent, as far as 

 regards their development, of either of those parts. Their minute structure at 

 an early period closely corresponds with that of the spleen and supra-renal 

 glands ; and the formation of their vesicles takes place after precisely the same 

 plan. This body, like the Supra-renal and Thymus, is of larger relative mag- 

 nitude during intra-uterine existence and infancy than in after life. 



488. That the Ductless Glands, of whose peculiar structure and relations we 

 have thus taken a general survey, have some office of importance to perform in 

 the preparation and maintenance of the Blood, cannot any longer be reasonably 

 questioned ; and the determination of this point may be fairly regarded as a 

 considerable step in the investigation. It is obvious, from the very copious 

 supply of blood which they receive during the period of their functional vigor, 

 and from the manner in which this is distributed by minute capillary plexuses, 

 on the exterior, and even through the interior, of the glandular vesicles, that 

 it must be subservient to some process of active change ; and the aspect of the 

 contents of these vesicles, as well as of the substance in which they are imbedded, 

 indicates that cell-growth is rapidly proceeding, at the expense of the materials 

 thus afforded. But, on the other hand, that the products of this cell-growth 

 are not substances which, like those of the ordinary glands, must be separated 

 from the Blood, either for its purification, or to serve some special purpose in the 

 economy, appears from the fact that they are not carried off by ducts, but are 

 received again into the current of the circulation. This would be equally true 



1 See Mr. Simon's admirable "Physiological Essay on the Thymus Gland," from which 

 the foregoing summary has been derived. 



2 That the fluid does not contain true Albumen in solution, but some albuminous com- 

 pounds, is indicated by the results of Mr. Beale's analysis ("Cyclop, of Anat. and 

 Physiol.," vol. iv. p. 1106). 



