GENERAL REVIEW OF THE NUTRITIVE PROCESSES. 567 



Thymus gland diminishes in proportion to the diminished activity of the gene- 

 ral nutritive functions; and that its fluid decreases, andjDecomes less rich in 

 globules, when the supply of food is insufficient, or the animal is prevented by 

 other causes (such as over-fatigue) from duly assimilating it. At present, the 

 question of the specific use of this body must be left in uncertainty. It may 

 be surmised, however, from the very striking analogy which it bears to the 

 Spleen, that its function is double; and that, whilst it serves as a diverticulum 

 for blood, it also performs some office in connection with the Lymphatic system. 

 It would be interesting to examine if, in those animals in which the Spleen 

 has been extirpated, the Thymus Gland undergoes any increase in size. 



713. The Thyroid Gland appears to have a structure analogous to that of 

 the Thymus ; but less is known respecting it. When incised, small cells may 

 be detected in it : but no connection has yet been traced between them, nor is 

 there any common reservoir. Like the Thymus, this body has no excretory 

 duct, but is provided with large Lymphatics, which directly enter the great 

 veins.* Though proportionally larger in the foetus than in the adult, it remains 

 of considerable size during the whole of life, and is supplied with arteries of 

 large calibre. The fluid which the cells contain is viscid and nearly colour- 

 less, sometimes having a yellowish tinge ; when put into rectified spirit, it 

 becomes solid but not opaque ; and it probably contains, therefore, some modi- 

 fication of albumen. The Thyroid body is the seat of that enlargement of the 

 neck which is known as goitre or bronchocele. In the commencement of this 

 disease, there seems at first to be simply distension of the cells resulting from 

 increased secretion ; so that, when the body is cut into, no change of structure 

 is observed in it but such as results from the enlargement of the cells, which 

 are of various sizes, usually from that of a pea downwards, and are filled with 

 a more or less viscid fluid. In more advanced cases, however, other alterations 

 are commonly met with, resulting probably from the altered vascular action 

 generated by the primary affection ; so that, when the tumour is cut into, stea- 

 tomatous, cartilaginous, or even ossific deposits are found in it. 



CHAPTER XIII. 



GENERAL REVIEW OF THE NUTRITIVE PROCESSES.ANIMAL HEAT. 



I. Review of the Nutritive Processes, with Practical Applications. 



714. THE detailed survey which has been now taken of the different Func- 

 tional operations concerned in maintaining the life of the individual, may sug- 

 gest to us some general views that have important practical applications. In 

 the first place, it has been shown, that the province of the Animal is not to 

 combine Inorganic elements into Organic compounds, fit to be applied to the 

 purposes of Nutrition ; but to use those which are prepared for it by the Plant. 

 The nutritive materials thus obtained may be divided into two great classes, 

 the azotized and the non-azotized. The former have been shown ( 457) to 

 be so nearly identical in composition with the proximate principles of which 



* See King, in Guy's Hospital Reports, vol. i. 



