THE HUMAN SPECIES 



The accessory glands connected with the respiratory system, 

 namely, the thyroid and thymus glands, present no important 

 points of difference in man and the mammals. Of the thyroid 

 gland we know that it arises through segmentation of a part of 

 the anterior wall of the primitive intestinal tube (Remak) in 

 the form of a plexus of vesicles, which are lined with epithelium 

 and sometimes appear in isolated groups, sometimes united in 

 a single mass. In the lower mammals (monotremata, many 

 marsupials and certain members of other classes) two distinct 

 glands are found, one on either side of the larynx ; in other 

 mammals, as in man, the two lateral masses are connected by 

 means of a bridge. 



The diameter; of the follicles of the thyroid gland is: 



= 0-045 - O'i mm. 

 the horse = 0x350 - 0-4 

 cattle = 0x335 - o - 2 

 the pig = 0-055 - 0-5 ! 



The origin of the thymus 

 gland is obscure. In man and 

 animals this organ is constructed 

 of glandiform follicles and divided 

 into large and small lobes ; in all 

 vertebrates it is most fully de- 

 veloped in youth and is, with rare 

 exceptions, vestigial in the adult. 

 In man it consists of two lobes 

 unequal in size only in the em- 

 bryo, and undergoes reduction and 

 FIG. 65. Terminal ramifications disappearance, usually commenc- 



of the bronchial tubes with , r , , 



the air-cells. (Thome, Zool- mg towards the end of the second 

 year of life. 



VI. Digestive System. 



In order to form a correct estimate of the distinctive 

 characters of the human digestive system we must first examine 

 what man possesses, in this respect, in common not only with 

 the rest of the vertebrates but also with the invertebrates. 



1 M. Sussdorf in Ellenberger, loc. cit., p. 524. 



