CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 763 



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 haematoidin (bilirubin), has been 

 found. 



495. 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 muciri-like body in the alveoli, 

 and the tendency 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 convul- 

 sions (more especially observed in young animals), accompanied or 

 succeeded by irregularity or failure of voluntary movements ; subse- 

 quently 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 above kinds 

 of animal, no serious symptoms follow, even the total extirpation 

 of the organ producing no marked effect ; and in rabbits and other 

 herbivorous animals removal is said never to be followed by any of 

 the above results. It has been urged that the symptoms when seen 

 are the effects not of the mere absence of the organ but of mischief 

 set up by the operation in adjoining structures, more especially in 

 the laryngeal nerves and vagus trunks ; but this does not seem a 

 valid explanation. If, as suggested above, certain metabolic pro- 

 cesses are normally going on in the organ, we may fairly suppose 

 that, in the absence of the organ, the interruption of the normal 

 sequence of chemical change would throw upon the circulation 

 certain strange substances which acting like a poison might pro- 

 duce the nervous symptoms, throw into disorder the nutrition of 

 various tissues, and finally bring about death. We may further 

 explain the cases where symptoms are absent by supposing that, 

 for some reason or other " things have taken a different turn ", the 

 particular poisonous substances have not made their appearance but 

 innocuous ones have taken their place ; and we know how slight a 

 change in chemical composition may turn a poison into an inert 

 body. This of course remains a mere supposition until we can state 

 what the exact metabolic processes are, and name the substances 

 which work the mischief; but it seems more reasonable to accept 



