ON THE ANATOMY OF THE TIIREE-TOED SLOTH. 157 



7. A Contribution to the Anatomy of the Three-toed Sloth 

 (Bradypus tridactylus). By Ghas. F. Sonntag, M.D., 

 F.Z.S., Anatomist to the Society. 



[Received November 16, 1920 : Read February 8, 1921.] 



(Plates I.-IY. & Text-figures 10-15.) 



The observations recorded here were made on the body of a 

 female Bradypus tridactylus which died a few hours after admis- 

 sion to the Society's Gardens. It was deposited, so I was unable 

 to make a complete dissection, and my investigations were, in 

 consequence, limited to the month, tongue, and organs of the 

 thorax and abdomen. Some of the conditions observed have not 

 been recorded before, and others differ from those which have 

 already been described. The organs were compared with 

 those of B. gularis or cuculliger, which are preserved in the 

 Museum of the Royal College of Surgeons. 



The Mouth. 



The Vestibule. 



The form and communications of the vestibule depend on the 

 shape of the maxillse and the size and distribution of the teeth. 

 When the skull is examined it is seen that the premaxillse are 

 edentulous, so there is a gap in front between the first pair of 

 teeth in the upper jaw (Plate I. C), and there is a similar gap in 

 the lower jaw. Consequently, when the lips are everted, one can 

 see the tip of the tongue. In Cholocpus, on the other hand, the 

 prominent rostrum, or beak, of the lower jaw diminishes the gap 

 in the centre. 



The mucous membrane does not form a vestibular pocket in 

 front, as in most mammals, for it passes direct from the margins 

 of the jaws to the free edge of the lips. Consequently, the 

 vestibule in front has no wider area than the orifice of the lips 

 (Plate I. A). 



The first pair of upper teeth are small, and the second pair are 

 large and project laterally (Plate I. C). Between the mid-line 

 in front and the second pair of teeth the upper jaw expands, but 

 it narrows behind them, and, as the cheeks are not closely applied 

 to the sides of the jaw, the vestibule is patulous and increases in 

 width from before backwards (Plate I. B ; v). In the case of the 

 lower jaw, however, the cheeks are closely apposed against the 

 gum and the vestibule is reduced to a mere slit. 



Posteriorly the communications between the vestibule and 

 mouth-cavity behind the last pairs of teeth in both jaws are 

 exceedingly small. 



In Cholcepus the vestibule and mouth-cavity communicate 

 through the gaps between the first and second pairs of teeth in 

 both Jaws (Plate I. D). 



