SENSE ORGANS. 



323 



crocodiles and Sphenodon, and slightly bent, thus resemblmg 

 that in birds and foreshadowing the si3iral cochlea of mammals. 

 In many reptiles the ductus endolymphaticus (aqueductus 

 vestibuli) ends in the cranial cavity just beneath the skull roof, 

 and in the Ascalahota it extends from the skull cavity into the 

 neck, swelling into a large lobed sack in the neighbourhood of 

 the shoulder girdle. It is filled with a soft otolithic mass. 



The patches of sensory epithehum in the membranous labyrinth, where 

 the branches of tlie auditory nerves end, are as follows : (1) in each ampulla 

 there is a projection of sensory epithelium known as the crista acushca ; 

 (2) there is a patch in the saccule and utricle known as the macula 

 acustica of the saccule and vitriele respectively ; (S) on the floor of the 

 utricle there is an additional patch, the macula acustica neglecta ; (4) in the 

 cochlear there are two patches, the papilla acustica hasilaris and the 

 papilla acustica lagenae. In the Chelonia and Ophidia, the cochlear process 

 is hardly differentiated into a pars hasilaris next the utricle and a peripheral 

 lagena, but in the Crocodilia the pars hasilaris is well developed and forms 

 the main part of the ductus cochlearis, the lagena being merely a terminal 

 end-sac. The papilla acustica hasilaris which is contained in the pars 

 hasilaris is not differentiated into the organ of Corti, but in crocodiles the 

 membrane on which it is placed is called the membrana hasilaris and there 

 is an indication of the scala vestibidi, scala ty^npani and membrane of 

 Reissner. * 



A tympanic cavity, membrane and eustachian tube are present 

 except in snakes and apodal lizards. There is a fenestra rotunda 

 as well as a fenestra ovalis, and the columella auris passes from 

 the latter to the tympanic membrane (for details see account of 

 orders). A cutaneous fold above the tympanic membrane of 

 crocodiles rej)resents the first trace of an external ear. 



The olfactory organ presents, particularly in the Chelonia 

 and Crocodilia, a considerable augmentation of the surface of 

 the nmcous membrane, the folds of which are supported by the 

 single cartilaginous turbinal. The lacrymal duct usually opens 

 beneath the turbmal, but it may open into the posterior nares 

 [Ophidia) or into the pharynx [Ascalahota). 



Jacobson's organs are absent in Crocodilia and Chelonia. 

 In Lacertilia and Ophidia they are present between the nasal 

 sacs and the roof of the mouth (between the turbinals and 

 vomer) as a pair of sacs lined by olfactory epithelium and open- 

 ing into the mouth just in front of the choanae. They develop 

 as outgrowths of the nasal sacs, are innervated by the olfactory 



* G. Retzius. Das Gehorogan d. Wirbelthiere, vols. 1 and 2, Stockholm, 

 1881 and 1884. 



