THE DIGESTIVE AND RESPIRATORY SYSTEM 289 



ev aginations that in the latter class arise from the same region 

 ai.d become eventually lost in the lobes of the thyreoid gland, 

 the so-called parathyreoid bodies. Comparing the significance 

 of the post-branchial bodies, they are looked upon by some as 

 the rudiments of a pair of pharyngeal pockets posterior to the 

 last definite ones, a point of view which has suggested for them 

 the name of ultimo -branchial (rather than post-branchial) 

 bodies ; but the probability that the last apparent pocket in the 

 various vertebrate Classes is not always the same one intro- 

 duces a valid objection to this hypothesis. 



Still another organ associated with the pharynx as primarily 

 an evagination from its walls is the thyreoid gland. This, 

 in its first appearance, is constant from the selachians to the 

 mammals, and arises as a median outpushing from the floor 

 of the pharynx at a level corresponding to the interval be- 

 tween the first and second pockets. In its later development 

 it becomes, like the thymus, a compact glandular organ, with- 

 out a duct, and equally uncertain and indefinite in its function, 

 but in the larva of one of the cyclostomes (Petromyzon), we 

 catch a glimpse of its past history, for here for a time it ap- 

 pears as an open trough, lined with cilia, and in open com- 

 munication with the pharynx. In this condition it corresponds 

 closely in structure and position with the hypo-branchial 

 groove, or endostyle of Amphioxus, an organ which furthers 

 the passage of the food down the pharynx by producing a 

 slimy secretion and by furnishing a ciliated track along which 

 the food may be propelled. The thyreoid gland was thus 

 primarily a digestive organ and deserves a place in this chap- 

 ter for reasons more fundamental than a mere topographical 

 relation. In the true vertebrates, however, its function, with 

 its structure, becomes completely changed, and it no longer 

 assists in digestion, but presumably possesses some regulating 

 effect upon the blood, especially that supplying the brain. 

 There is yet much that is problematic in this, but that the 

 association between the thyreoid and the brain is an intimate 

 one is shown both by the occurrence of cretinism, a curious 

 developmental malformation associated with the thyreoid and 



