36 NATURAL HISTORY OF VERTEBRATES. 



closed ill the adult. The original opening by which the cavity of the labyrinth com- 

 municates with the outside is closed in the adults of almost all vertebrates, but the 

 sharks form an exception to this rule, the endolymphatic duct, as it is called, remaining 

 permanently open. This duct is so named on account of the fluid endolymph contained 

 in the labyrinth, the vibrations of which disturb the hairs projecting from the sensory 



cells into this fluid. The sensory cells are only accumu- 

 lated at certain spots in the labyrinth, to which the 

 branches of the auditory nerve are distributed ; three of 

 these are known as ' cristse acusticse,' and project into the 

 ' ampullae,' which terminate one end of each of the three 

 semicircular canals, while tliree others are larger patches, 

 ' maculae acusticae,' which occupy the recess of the utricu- 

 sa lus, the sacculus, and the cochlea respectively. Resting on 



oat^flsii, right side Lner'aspect'; these three maculsB are certain ear stones or otoliths formed 

 recessL?itrUu'u';aa!"t«p%m^ °^ <^ense calcareous matter, and known as the lapillus, 

 anS posterta- temMrouter^can- sagitta, and astcriscus. Like the maculae they vary much 

 oMifh^fa7Y'&ieni''^^cU^ in form and relative size throughout the class of the fishes. 

 Sricuit'saiciCiuot, indfcate? Of the semicircular canals, two^are vertical, while the third 

 ettv?c*ts'''a^commun"oIS *rith (external) is horizontal in position ; the anterior of the ver- 

 inttfs''lnd'som°e'otiitr''fishe3^''*'^ ^^'^'^^ canals is approximately in a sagittal plane, the pos- 

 terior in a frontal plane. 

 The wall of the labyrinth is formed of a very delicate cartilage, lined by a low 

 epithelium, except at the spots where the sensory cells are accumulated. A quantity 

 of loose tissue containing fluid (perilymph) surrounds the labyrinth, and serves to 

 transmit vibrations to the latter. The chief channel by which vibrations reach the ear 

 in fishes is unquestionably the loose bones of the head and gill-cover, but there may 

 be other channels, notably the air-bladder in the cat-fish and sucker, and their allies, 

 while in some sharks the first gill-cleft apparently serves for this purpose. 



The higher Vertebi-ata not only approach the sharks more closely in this respect 

 than they do the ordinary fishes, but also in respect of the complete nature of the 

 capsule surrounding the labyrinth. This capsule, at first cartilaginous, afterwards 

 ossifies, and forms the osseous labyrinth, which agrees in all respects with the enclosed 

 ' membranous ' labyrinth in form, there being only a very narrow space, containing peri- 

 lymph, left between the osseous capsule and its contents. The only deficiencies in the 

 wall of the osseous labyrinth, apart from the aperture by which the nerve gains admis- 

 sion, are two holes on the lateral aspect, which look into the tympanic cavity or cavity 

 of the drum of the ear, present, with few exceptions, among the higher vertebrates. 

 One of these holes, the ' foramen ovale,' is opposite the sacculus, and is occupied by the 

 inner end of the chain of bones that stretches across the tympanic cavity, while the 

 other corresponds to the cochlea in position, and, being occupied by a tense mem- 

 brane, is known as the ' fenestra rotunda.' It is chiefly through the chain of bones 

 that sound-waves set the perilymph, and consequently the labyrintli which floats freely 

 in it, in motion ; although vibrations are no doubt also transmitted through the bones 

 of the head. 



Before referring more specifically to the method ' by which vibrations are trans- 

 mitted to the labyrinth in the higher vertebrates, one or two points in which the 

 labyrinth itself differs from that of the fishes must be referred to. In the first place, 

 the otoliths which are so conspicuous in the fishes are barely represented among the 



