THYROID GLAND. ASSIMILATING GLANDS IN GENERAL. 523 



tionably larger in the foetus than in the adult, it remains of considerable size 

 during the whole of life. It appears, from the recent inquiries of Mr. Simon,* 

 that a Thyroid gland, or some organ representing it in place and office, exists 

 in all Vertebrated animals. It presents its simplest form in the class of Fishes ; 

 in some of which it appears to consist merely of a plexus of capillary ves- 

 sels, connected with the origin of the cerebral vessels, and capable, by its dis- 

 tensibility, of relieving the latter, in case of any obstruction to the proper 

 movement of blood through them. In the higher forms of this organ, the 

 glandular structure, consisting of closed vesicles over which the capillary 

 plexus is distributed, and of their cellular contents, is superadded ; and the 

 organ then appears, like the Spleen, to be destined for two different uses ; 

 namely, to serve as a diver ticulurfi, to the Cerebral circulation ; and to aid in 

 the elaboration of nutritive matter, which is taken up by the Absorbent sys- 

 tem, and which is again poured by it into the general current of the circula- 

 tion. 



690. Thus the Spleen, the Supra-Renal Capsules, the Thymus Gland, and 

 the Thyroid Gland, all seem to share in the preparation of the nutritive ma- 

 terials of the blood, along with the ordinary glandulse of the Absorbent system. 

 In fact, we may regard them all as together constituting an apparatus, which 

 is precisely analogous to that of the ordinary glands, but of which the element- 

 ary parts are scattered through the body, instead of being collected into one 

 compact structure. Thus if we could imagine any tubular gland, such as the 

 Kidney or the Testis, to be unravelled, and its convoluted tubuli to be spread 

 through the system, yet all discharging their contents by a common outlet, we 

 should have no unapt representation of the Lymphatic portion of the Absorb- 

 ent system. Its function appears to be, to separate the crude Albuminous 

 matter from the blood, to subject it to an elaborating action performed by the 

 epithelium-cells lining the tubes, and then to pour forth this elaborated pro- 

 duct, not as an excretion to be carried out of the body, but (in conjunction 

 with that, which has been newly taken in by the Lacteal portion of the sys- 

 tem, and which has undergone elaboration by its glandulae), into the blood- 

 vessels, which are to convey it to the different parts of the body where it is 

 to be appropriated. The four bodies we have been just considering, appear 

 to be, so far as their glandular function is concerned, appendages to this sys- 

 tem. Their uses as diverticula to the circulation through other organs, render 

 them liable to occasional distention with blood; and it seems determined that 

 this blood shall not lie useless, but shall be subservient to the action in ques- 

 tion ; the gland-cells that line the cavities of the organ withdrawing certain 

 constituents of the blood, to restore them, through the Lymphatic system, in 

 a state of more complete preparation for the operations of Nutrition. Their 

 function is very probably vicarious ; that is, the determination of blood is 

 greatest (through the state of the other organs) at one time to one of these bodies, 

 and at another time to another. Hence the effects of the loss of any one of 

 them are not serious ; as the others are enabled in great degree to discharge 

 its duty. 



4. Composition and Properties of the Chyle and Lymph. 



691. The chief chemical difference between the Chyle and the Lymph, 

 consists in the much smaller proportion of solid matter in the latter, and in the 

 almost entire absence of fat, which is an important constituent of the former. 

 This is well shown in the following comparative analyses, performed by Dr. 

 G. O. Rees, of the fluids obtained from the lacteal and lymphatic vessels of a 



* Philosophical Transactions, 1844. 



