772 DEVELOPMENT OF THE ORGANS OF SENSE. 



inner forms the external walls of the membranous labyrinth, while the 

 interveninp; layer swells up into gelatinous tissue, the meshes of which 

 become wider and wider, till at last the space is left which ultimately 

 contains the perilymph. 



The cochlea appears at first as a prolongation downwards from the 

 auditory vesicle, but afterwards becomes tilted forwards. This pro- 

 longation of the auditory vesicle is the rudimentary canalis mem- 

 branacea. Close to it is placed the cochlear nerve, with a gangliform 

 extremity. The canal becomes elongated in a spiral direction, and the 

 ganglion, which is elongated with it, becomes the ganglion spirale. 

 Between the canal and the cartilaginous wall which afterwards sur- 

 rounds it a large amount of connective tissue intervenes, and in this 

 tissue the cavities of the scala vestibuli and scala tympani gradually 

 appear at a later period, precisely as does the space for the perilympli, 

 in the vestibule. The modiolus and spiral lamina, according to Kul- 

 liker, are ossified without intervention of cartilage. Within the canalis 

 membranacea Kulliker finds in the embryo a continuous epithelial 

 lining, thin on the membrane of Reissner and on the outer wall, 

 but forming a thick elevation in the position of the rods of Corti, 

 and a large'r elevation more internally, filling up the sulcus spiralis. 

 On the surface of this latter elevation he has observed a transparent 

 body, the membrane of Corti. 



The auditory nerve is said to be developed, separately from both the brain and 

 the labyrinth, in the intermediate mesoblastic wall of the head ; the canal termed 

 meatus auditorius internus being left in the bones round it and the facial nerve. 

 The auditory nerve becomes secondarily connected with the medulla obloug-ata 

 in a manner not yet ascertained, and its fibres are extended into relation with 

 the delicate terminal structiu-es formed in the membranous labyrinth. 



Middle and External Cavities of the Ear. — It has been already explained 

 that the external meatus, the tympanic cavity, and the Eustachian tube, are 

 formed in the posterior or upper part of the first postoral visceral cleft, which 

 remains permanently open as the tympano-eustachian passage, except at the place 

 where it is interrupted by the formation of the membrana tjmipani ; and also 

 that the malleus is formed in the first visceral plate from the proximal part of 

 Meckel's cartilage, and the incus, stapes, and stapedius muscle and the styloid 

 process probably in the second plate. It is pointed out by KoUiker that during 

 the whole period of foetal life the tympanic cavity is occupied by connective 

 tissue, in which the ossicles are imbedded ; and that only after respiration has 

 been established this tissue recedes before an expansion of the mucous membrane. 



The pinna is gradually developed on the posterior margin of the first visceral 

 cleft. It is deserving of notice that congenital malformation of the external 

 ear, with occlusion of the meatus and greater or less imperfection of the tympanic 

 apparatus, are observed in connection with abnormal development of the deeper 

 parts of the first and second visceral plates and the intermediate cleft ; while 

 cases have been observed of the persistence in the neck of the adult of one or 

 more of the branchial clefts situated behind the first. (Allen Thomson, Proceed. 

 Eoy. Soc. of Edin. 1844, and Edin. Journ. of Med. Sc. 1847.) 



DEVELOPMENO? OF THE NOSE. 



The organ of smelling, as was first pointed out by V. Baer, owes its 

 origin, like the primary auditory vesicle and the crystalline lens of the 

 eye, to a depression of the integument, or what may be more precisely 

 designated as epiblast. This depression, the primary olfactory groove, 

 is at first encircled by a uniform wall, and is unconnected with the 



