744 COMPARATIVE ANATOMY 



cardial bodies because they secondarily become associated with the peri- 

 cardium of the selachians. At present they are usually called post- 

 branchial bodies. In selachians a complete pair of these bodies develops, 

 but in urodeles and lizards only the left one ever completes its develop- 

 ment, the right ultimately disappearing. Whether these bodies occur in 

 birds and mammals is not known, although there are somewhat similar 

 growths which are called parathyroid bodies, which do develop in these 

 animals and then become lost in the lobes of the thyroid gland. 



Explanations of these bodies are not yet satisfactory. 



The thyroid has come to be considered a very important organ since 

 endocrinology looms up so large in the medical world. This gland is 

 an evagination of the pharynx. It is first seen in the selachians, but 

 makes its appearance regularly in the higher forms. It arises from the 

 floor of the pharynx at about the level of the interval between the first 

 and second pockets. It becomes compact, and, like the thymus, does 

 not develop a duct. In the larva of Petromyzon (one of the cyclostomes) 

 the thyroid appears as an open trough, lined with cilia, which is in open 

 communication with the pharynx, a position quite like that in Amphi- 

 oxus. This trough is called the hypo-branchial groove or endostyle, an 

 organ which assists the passage of food down the pharynx by exuding 

 a slimy secretion and by furnishing a definite track, with cilia, which 

 can thus facilitate its movement. 



Professor Wilder thinks the thyroid gland is primarily a digestive 

 organ, though in the true vertebrates its structure, as well as its func- 

 tion, has nothing to do with digestion. It is now generally taught 

 that the internal secretions of the thyroid gland stimulate growth and 

 inhibit development, while the secretions of the thymus gland stimulate 

 development and inhibit growth. 



THE OESOPHAGUS 



This is the swallowing tube connecting the mouth with the stomach. 

 It lies directly against the interior of the dorsal w r all of the body-cavity 

 and thus lacks a serous coat. There are no digestive glands in its walls 

 as a rule. Its length quite naturally varies with the length of the neck 

 of the animal in which it occurs. Usually, its internal lining is a smooth 

 epithelium. In the chelonians one finds cornified papillae pointing back- 

 wards. The oesophagus, like other parts of the digestive tract, consists 

 of five layers ; however, as it will be remembered that the ectoderm has 

 indented to form the anterior and posterior openings into the digestive 

 tract, a histological examination in the region where these two divisions 

 merge into each other will show a change of structure. 



The muscles contained in the walls are striated at the cephalic end 

 and extend back in some cases even into the stomach. The oesophagus 

 usually has the same diameter throughout, but in many, if not most 

 birds, there is a dilation called the crop or ingluvies. This may either 



