ARREST OF DEVELOPMENT. 91 



median incisors ; the upper specimen apparently con- 

 sists of the two median upper incisors intimately united. 

 They are probably all from the same subject. 



Presented by J. Swan, Esq., 1838. 



392. A Human second molar tooth with an inverted tooth 

 wedged in between its fangs. (The inverted tooth is 

 probably the wisdom-tooth, which has been developed on 

 the ascending ramus of the mandible, and which has 

 grown downwards, displacing the fangs of the second 

 molar with its crown and becoming impacted between 

 them.) 



399. The double tusk of an Elephant. 



B. Of the Alimentary Canal. 



Class V. AEREST OF DEVELOPMENT*. 



a. Of the pharynx and oesophagus. 



The pharynx may terminate in a blind pouch, which is either 

 entirely separated from the oesophagus or connected with it by 

 a fibrous or muscular cord only ; in such cases the oesophagus 

 often opens into the trachea, near its bifurcation. 



394. The tongue, pharynx, trachea, and oesophagus of a Human 



foetus, with a small portion of the diaphragm and 

 stomach. The oesophagus opens into the trachea just 

 above its bifurcation; it is not connected with the 

 pharynx. The trachea has been opened in front, and 

 a green glass rod has been inserted into the oeso- 

 phagus. 



395. A similar preparation from a new-born Child, in which the 



lungs and stomach are also preserved. The oesophagus 

 is united with the pharynx by a muscular band. 



Presented by Dr. Ay res. 



396. A similar preparation from a Human foetus ; the lungs are 



abnormal in form, and have not been inflated with air. 



b. Atresia of the rectum. 

 Syn. Atreto cormus, Ourlt. 



One of the most frequent malformations which come under 

 * The other Classes have not occurred in this Series. 



