460 THE METABOLIC PROCESSES OF THE BODY. 



middle and lower cervical ganglia; their exact terminations within the 

 organ is not known. Fine filaments are also said to be given off to it from 

 the external branch of the superior laryngeal nerve. 



The " accessory " thyroid bodies often found are of the same nature as 

 the main body. 



Very frequently, so frequently in the adult as to be of almost normal 

 occurrence, the alveoli contain not simple glairy fluid but a more solid clear 

 material called " colloid ;" this generally appears in the centre of an alveolus 

 and may fill up the whole lumen ; occasionally more or less changed epi- 

 thelial cells may be seen lying between it and the layer of cells resting on 

 the basement membrane. Extravasations of blood into the alveoli are also 

 not uncommon. 



The thyroid body is very apt to become enlarged, sometimes enor- 

 mously so, and is then spoken of as goitre. The enlargement may be due 

 simply to an increase in the number of otherwise fairly normal alveoli and 

 septa. But very often a number of alveoli become more or less confluent, 

 forming a cyst ; and at times the whole gland appears to be composed of a 

 number of cysts of varying size, frequently loaded with "colloid" material. 

 There is also a form of goitre in which the enlargement is chiefly or even 

 exclusively due to an increase in the vascular supply, the bloodvessels being 

 abnormally distended ; and this apparently may occur without any structural 

 changes in the walls of the bloodvessels. Sometimes, however, the arteries 

 undergo aneurismal enlargements, with changes in their coats. 



The glairiness of the fluid contents of the alveoli has generally been 

 attributed to the presence of mucin, and this body has also been said to 

 have been found within the lymphatic vessels running in the septa; but 

 some observers have urged that the material in question is not true mucin, 

 but a peculiar form (or forms) of proteid substance. The " colloid " material 

 so frequently appearing has also been regarded as allied to mucin, but its 

 exact nature has not as yet been satisfactorily determined. Besides these 

 special substances the alveoli or cysts also contain serum-albumin and 

 globulin. The " extractives " of the thyroid appear to contain kreatin or 

 kreatinin in not inconsiderable quantities, xanthin, and lactic (paralactic) 

 acid ; guanin is said to be absent. In large and old cysts cholesterin is 

 sometimes present ; and when, as often happens, extravasations of blood into 

 the cysts have taken place, haemoglobin, or at a later stage hsematoidin 

 (bilirubin) has been found. 



409. The large supply of blood to the thyroid suggests the idea that 

 the organ is the seat of some of the subsidiary metabolic processes to which 

 we referred in the last section, and this view is supported by the presence of 

 the extractives just mentioned ; but we have no detailed knowledge of what 

 actually goes on. 



The presence of the peculiar mucin-like body in the alveoli, and the ten- 

 dency to " colloid formation " further suggest? some relation of the organ to 

 the formation or distribution of mucin ; and this view has derived a certain 

 support from some experimental results, but these, though numerous, have 

 proved neither uniform nor accordant. When in certain animals (monkeys, 

 dogs, and other carnivora, and the same has been observed in man) the gland 

 is extirpated, even with the greatest care, the operation is frequently followed 

 by the occurrence of peculiar nervous symptoms, such as muscular twitchings 

 and tremors, spasms, and even tetanic convulsions (more especially observed 

 in young animals), accompanied or succeeded by irregularity or failure of 

 voluntary movements ; subsequently there may ensue varied symptoms 

 which may be described under the general term of disordered nutrition, 

 ending eventually in death. In a certain number of cases, however, in the 



