THE ALIMENTARY CANAL AND ITS APPENDAGES 163 



The whole organ at first has an undoubtedly glandular 

 character, but after the constriction is completed it undergoes a 

 marked structural change. 



The manner in which the thyroid originates justifies us in 

 classing it as a vestigial organ. In the further course of its 

 development, however, it does not degenerate as might be 

 imagined CL priori ; on the contrary, it develops into a large, 

 highly vascular organ, which, according to recent clinical experi- 

 ence, iifoTgreat service in the maintenance of both the bodily 

 and mental health of its possessor. 



It would appear to play some important function in relation 

 to the central^ nervous system, since its removal in animals is 

 attended with the manifestation of an extraordinary number of 

 pathological symptoms, idiocy, muscular twitchings, tetanic, 

 ataxic, apathic, clonic, and epileptic symptoms being conspicuous, 

 with marked disturbances of the organs of deglutition, circulation, 

 and respiration (cachexia strumipriva). It may further be noted 

 that different classes of animals are differently affected by the 

 destruction of this organ. 1 



This gland may be concerned either in the production of a 

 secretion, or in the removal from the blood of substances which 

 would be injurious to the nervous system ; but nothing very 

 definite is known concerning its functions. It is richly supplied 

 with blood, indeed much more so than the brain itself. 



In the thyroid gland, then, we have evidence of change of 

 function, and this is also the case, at least to a certain extent, 

 with the thymus. In Mammals, and especially in Man, this 

 gland is chiefly formed from a hollow epithelial outgrowth of the 

 third_J)ranchial pouch, although the fourth, and to a certain 

 extent the second also, take part in its formation. 



The thymus thus far resembles in its origin a gland ; but it 

 loses this character, and a thorough histological change takes 

 place in consequence of the wandering into it of lymphoid cells. 

 This change renders its physiological significance still more 

 difficult to explain. Towards the end of the second year the 

 thymus (the greater part of which now lies behind the sternum, 

 i.e. ventrad of the heart and of the roots of the larger blood- 

 vessels) reaches its highest development, and after that period it, 

 as a rule, undergoes retrogressive metamorphosis ; in very old 



1 It is difficult to decide whether and to what extent the frequent pathological 

 affections of the thyroid gland (the formation of a "crop" with secondary disorgan- 

 isation of the tissues) may or may not be referred to change of function within it. 



