928 THE ORGANS OF XENSE. 



bottom of the internal auditory meatus, into two branches, the cochlear and ves- 

 tibular. The trunk of the nerve, as well as the branches, contains numerous 

 ganglion-cells with caudate prolongations. 



The vestibular nerve, the posterior of the two, divides into three branches 

 superior, middle, and inferior. 



The superior vestibular branch, the largest, divides into numerous filaments, 

 which pass through minute openings at the upper and back part of the cul-de- 

 sac at the bottom of the meatus, and, entering the vestibule, are distributed 

 to the utricle and to the ampulla of the external and superior semicircular 

 canals. 



The middle vestibular branch consists of numerous filaments, which enter the 

 vestibule by a smaller cluster of foramina placed below those above mentioned, 

 and which correspond to the bottom of the fovea hemispherica ; they are distributed 

 to the saccule. 



The inferior and smallest branch passes backward in a canal behind the fora- 

 mina for the nerves of the saccule, and is distributed to the ampulla of the pos- 

 terior semicircular canal. 



The nervous filaments enter the ampullary enlargements at a deep depression 

 seen on their external surface, with a corresponding elevation when seen from 

 within ; the nerve-fibres ending in loops and in free extremities. In the utricle 

 and saccule the nerve-fibres spread out, some blending with the calcareous matter ; 

 others, radiating on the inner surface of the wall of each cavity, become blended 

 with a layer of nucleated cells and terminate in a thin fibrous film. 



The cochlear nerve divides into numerous filaments at the base of themodiolus, 

 which ascend along its canals, and then, bending outward at right angles, pass 

 between the plates of the bony lamina spiralis, close to its tympanic surface. 

 Between the plates of the spiral lamina the nerves form a plexus which contains 

 ganglion cells forming the ganglion spirale. From this ganglion delicate filaments 

 pass between the layers of the osseous lamina to the sulcus spiralis and pass out- 

 ward to the organ of Corti. Their exact termination is uncertain. Waldeyer 

 describes them as collected into two groups, one group ending in the outer and the 

 other in the inner hair-cells. 



The bottom of the internal auditory meatus, known as the lamina cribrosa, is 

 subdivided by a horizontal ridge, the crista falciformis, into a superior and an 

 inferior fossa. In the superior fossa is seen anteriorly the for amen faciale or 

 orifice of the aqueductus Fallopii ; and posteriorly is a group of foramina, area 

 cribrosa superior, for the nerve-filaments to the utricle, superior and external 

 semicircular canals (superior vestibular branch). In the inferior fossa are : (1) 

 a group of foramina, area cribrosa media, for the filaments to the saccule (middle 

 vestibular branch) ; (2) posteriorly, the foramen singulare, for the nerve to the 

 posterior semicircular canal (inferior vestibular branch) ; (3) antero-inferiorly, the 

 foramina for the filaments of the cochlear branch, grouped in a spiral, tract us 

 spiralis for aminulentus, and at the end of the spiral is the foramen centrale coch- 

 lea? or orifice of the central canal of the modiolus. 



Surgical Anatomy. Malformations, such as imperfect development of the external parts, 

 absence of the meatus, or supernumerary auricles, are occasionally met with. Or the pinna may 

 present a congenital fistula which is due to defective closure of the first visceral cleft, or rather 

 of that portion of it which is not concerned in the formation of the Eustachian tube, tympanum, 

 and meatus. The skin of the aui-icle is thin and richly supplied with blood, but in spite of this 

 it is frequently the seat of frost-bite, due to the fact that it is much exposed to cold, and lacks 

 the usual covering of subcutaneous fat found in most other parts of the body. A collection of 

 blood is sometimes found between the cartilage and perichondrium (licematoma auris), usually 

 the result of traumatism, but not necessarily due to this cause. It is said to occur moet fre- 

 quently in the ears of the insane. Keloid sometimes grows in the auricle around the puncture 

 made for earrings, and epithelioma occasionally affects this part. Deposits of urate of soda are 

 often met with in the pinna in gouty subjects. 



The external auditory meatus can be most satisfactorily examined by light reflected 

 down a funnel-shaped speculum; by gently moving the latter in different directions the 

 whole of the canal and membrana tympani can be brought into view. The points to be noted 



