180 AMPHIBIA. 



fourth ventricle. The cranial nerves have the same relations as in 

 the Fishes, since not only are the facial nerves and the nerves supply- 

 ing the muscles of the eye often connected with the trigeminal, but 

 the glossopharyngeal and the spinal accessory are represented by 

 branches of the vagus. The hypoglossal is, as in the Fishes, the first 

 spinal nerve. 



With regard to the sense organs the two eyes may be rudimentary 

 and concealed beneath the skin (Proteus -, Cceciliidca). In the Peren- 

 nibranchiata eye-lids are completely absent, while the Salamandrina 

 have an upper and lower eye-lid, and the Batrachians, except Pipa, 

 have, besides the upper eye-lid, a large very movable nictitating 

 membrane, with which a rudimentary lower eye-lid co-exists only in 

 Bufo. In the Batrachians there is a retractor muscle by means of 

 which the large bulb of the eye can be drawn back. The structure 

 of the auditory organ * of the Amphibia resembles that of Fishes. It 

 is usually confined to the labyrinth with three semi-circular canals ; in 

 the Batrachians alone there is a tympanic cavity, which communicates 

 with the pharynx by means of a wide Eustachian tube, and is closed 

 externally by a tympanic membrane, which is sometimes freely 

 exposed on the surface and sometimes covered by the skin. The 

 tympanic membrane is connected with the fenestra ovalis by a small 

 cartilaginous rod (remains of the hyomandibular) with a cartila- 

 ginous plate (columella with operculum). When there is no tympanic 

 cavity these structures are covered by muscles and skin. The cochlea, 

 which was first discovered by Deiters in the frog, is probably present 

 in all Amphibia. The olfactory organs are always paired nasal 

 cavities, which are provided with folds of the mucous membrane 

 and open internally either anteriorly within the lips, or, in the 

 Batrachians and Salamandrines, further back between the maxillaries 

 and palatines. The external skin, which is richly supplied with 

 nerves, is to be regarded as the seat of the tactile sense. The posses- 

 sion of the sense of taste is indicated by the presence of taste papillae 

 on the tongue of the Batrachians. The Amphibia certainly swallow 

 their food unmasticated, and the tongue also subserves other func- 

 tions ; for instance, in the Batrachia it is used as a prehensile organ. 



Alimentary canal. The mouth is a wide slit. The vomers, 

 palatines, and jaws are usually armed with sharp backwardly curved 

 teeth, which are used not for mastication, but for holding the prey. 

 Teeth are seldom absent, as in Pipa and some Toads ; but in the Frogs 

 they are always present on the upper jaw and palate. 



* Compare especially the works of Deiters, Basse, and Retzius. 



