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 «@ priori; on the contrary, it develops into a large, 
highly vascular organ, which, according to recent clinical experi- 
ence, is of great 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.? 
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 branchial 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. 
