290 HISTORY OF THE HUMAN BODY 



in which varying degrees of idiocy play a principal part, and 

 by the frequency with which insanity has followed the extir- 

 pation of the thyreoid. 



The pre-vertebrate history of the thymus-anlagen is less 

 certain, but attempts have been made to homologize them with 

 the " tongue bars" of Amphioxus, gelatinous rods used to 

 support the gill-clefts. If this be true the change of function 

 seems even greater than in that of the thyreoid, for there is 

 more similarity between a gland-lined groove and a compact 

 glandular organ, than between the latter and a skeletal rod. 

 In its development among the vertebrates the thymus is still 

 more problematical than is the thyreoid; it is usually volu- 

 minous, and in many mammals extends along the anterior 

 mediastinal space between sternum and heart as far as the 

 diaphragm. In the human species it is large in childhood, but 

 suffers regressive changes ; during middle life it is inconspicu- 

 ous, and in old age is reduced to a few rudiments. * /r - 

 1 ' Beyond the pharynx the alimentary canal becomes nar- 

 rowed and forms the oesophagus, below which it again en- 

 larges to form the great expansion known as the stomach, 

 usually the most conspicuous portion of the entire tract. The 

 oesophagus varies in length in proportion to the length of 

 the neck region, one extreme being represented by frogs and 

 toads, in which it is reduced to a simple constriction like the 

 neck of a bag, the other by such cases as a long-necked bircf. 

 In birds that are graminivorous, or those in general which sub- 

 sist upon hard or dry food, the middle portion of the oesoph- 

 agus expands to form a crop (ingluvies), into which tne 

 food is first collected. Aside from its use as a receptacle in 

 which food may become softened by soaking, the crop is 

 probably in part a provision for the safety of the birds, allow- 

 ing them to greatly shorten the period of feeding, a time dur- 

 ing which they are preoccupied and thus in especial danger 

 from their enemies. 



> The muscular fibers of the alimentary canal usually change 

 from voluntary to involuntary in the upper part of the 

 oesophagus, but in some mammals striated fibers, probably 



