276 CLASS AMPHIBIA. 



gives off a strong branch to the tongue which constitutes the 

 hypoglossal. In the frog the ganglion of the posterior root extends on to 

 the common stem, and the dorsal and ventral rami of the spinal nerves 

 both arise from it. 



The sympathetic is distinct and connected with the ventral rami of 

 the spinal nerves by rami communic antes. In the Urodeles it extends 

 along the whole length of the body into the caudal canal. In the Anura 

 it commences anteriorly in the skull as a cord leaving the ganglion 

 prooticum, receives a branch from the ganglion jugulare, and dilates in 

 the neck into the first ganglion of the sympathetic chain, which is con 

 nected by a ramus communicans with the first (second) spinal nerve 

 (Fig. 153). This is followed by nine ganglia, each of which is connected 

 with its spinal nerve by one, or in the case of the posterior ganglia by 

 more than one, ramus communicans. The tenth ganglion is small and 

 not always distinguishable. 



Sense organs. The nasal sacs are always provided with 

 internal nares. The eyes are reduced and covered by the skin 

 in the subterranean and cave-forms (Gymnophiona, Proteus, 

 Pipa). Eyelids, both upper and lower, are present in the 

 Salamandridae. but absent in other Urodeles. In the Anura 

 there is an upper eyelid and a movable nictitating membrane 

 which is sometimes called the lower eyelid. In Bufo there is in 

 addition a small lower lid. In the Anura there is a retractor 

 bulbi muscle by means of which the large bulb of the eye can be 

 drawn back. Lacrymal glands are absent, but there is an 

 Harderian gland in the inner angle of the orbit of the Anura 

 which opens within the nictitating membrane. There is a 

 fenestra ovalis (vestibuli) in all Amphibia, and in many Anura 

 a fenestra rotunda (cochleae) as well. There are three semi- 

 circular canals, and a small cochlea was discovered by 

 Deiters in the frog, and is probably present in other Amphibia. 

 The tympanic cavity and membrane are absent in Gymnophiona 

 and Urodela, and in some Anura they are much reduced or even 

 absent (Pelobatidae, Bombinator. Phryniscus, Batrachophrynus, 

 etc.). In most Anura they are present, though the membrana 

 tympani is not always visible on the exterior. In the Aglossa 

 the eustachian tubes join and have a single opening into the 

 pharynx ; in other forms they remain separate. The columella 

 auris extends from the fenestra ovalis to the tympanic membrane, 

 It lies outside the tympanic cavity, but projects into it. Its 

 internal end, called the stapes or operculum, fits into and fills- 

 the fenestra ovalis ; its outer end is connected with the membrana 

 tympani. In the Aglossa the outer end expands into a cartila- 



