600 AUDITORY NERVE. [SECT. 234. 



of the zona membranacea, in the entire extent of the cochlea. This 

 vessel, which is probably venous, always lies beneath the habenula 

 denticulata, sometimes more internally, sometimes more externally : 

 in the last half turn of the cochlea it is a capillary vessel, coo^" 

 in diameter; towards the base, however, it gradually reaches 

 o'oi^'" in breadth, and is distinctly composed of two coats. In 

 rare cases, there are two capillary vasa spiralia in the situation 

 mentioned, and Corti has twice found in man, and in the sheep, 

 an outer vas spirale also, near the ligamentum spirale on the zona 

 pectinata, but this did not communicate with the inner vessels, 

 the zona pectinata in general being non-vascular. 



We have still at the close to notice the auditory nerve. The 

 nerve-tubes of its trunk measure in man o - oo2'" to 0'005'", are 

 extremely readily destructible, and possess only a delicate neuri- 

 lemma. Between the tubules, numerous ganglionic cells are found, 

 in the trunk itself, and in the vestibular and cochlear nerves; 

 these cells are bipolar, apolar, or unipolar, pale or pigmented, and 

 measure o'02"' to o - 07"' in man and mammalia; those cells which 

 have only one pole, or none at all, are, probably, as Stannlus 

 suggests, only mutilated bipolar ones, seeing that the auditory 

 nerve, especially in fishes, may possess bipolar cells only, or 

 scarcely any others. Similar cells, but smaller, are also found, 

 as above mentioned, in the cochlea and on the nervous trunklets 

 in the vestibule (Pappenheim, Corti). Czermak has observed the 

 fibres of the auditory nerve to divide in the ultimate distribution 

 on the ampullae and sacculus of the sturgeon ; Heirless and myself 

 have seen the same thing in the frog, and Leydig in the cliimccra. 



With regard to the development of the organ of hearing, it may 

 only be mentioned here, that, according to Huschke's discoveries, 

 confirmed by Reissner and Remah, the membranous parts of the 

 labyrinth are formed from the external skin by a process of intro- 

 version, so that they might be compared in their origin to the 

 lens and the vitreous humour ; in this process, the layer of cells 

 corresponding to the epidermis is chiefly concerned, though not 

 exclusively, as Remah believes ; and to the introverted membrane 

 are superadded the auditory nerves from the brain, and the remain- 

 ing structures, both hard and soft, from the middle germinal 

 lamina, and thus the organ is completed. Nothing of importance 

 is known with regard to the histological development of the soft 

 parts of the labyrinth. 



For the investigation of the organ of hearing, which only in the labyrinth 

 presents very considerable difficulties, perfectly fresh preparations are indis- 





