116 



ZOOLOGY 



SECT. 



FIG. 789. Longitudinal section through ;ui ampulla, (t. e. auditory 

 epithelium ; a. h. auditory hairs ; c. part of semicircular canal ; 

 cr. crista acustica ; ct. connective-tissue ; t. i, epithelium ; n. 

 nei-ve ; u. junction with utricvilus. (From Foster and Shore's 

 Physiology.) 



otoliths of varying size and number. There is every reason for 

 thinking that the labyrinth, like the otocysts or statocysts in the 

 lower animals, functions as an organ of equilibration as well as of 

 hearing. 



As the membranous labyrinth develops in the embryo, it be- 

 comes surrounded and enclosed by the auditory capsule, the 



cartilage of which 

 adapts itself to the 

 form of the laby- 

 rinth, presenting a 

 large excavation 

 for the utricle and 

 saccule and tun- 

 nel-like passages 

 for the canals. The 

 auditory organ 

 does not, however, 

 fit tightly into 

 this system of 

 cavities, but be- 

 tween it and the 

 cartilage is a space, 

 filled by a fluid 

 called perilymph, 

 which acts as a buffer to the delicate organ floating in it. 



The early history of the auditory apparatus in the embryo 

 shows that it belongs to the same series of structures as the lateral- 

 line system, of which it may be regarded as a highly specialised part. 

 Nerve-components. The nerve-fibres of which the nerves 

 cerebral, spinal, and sympathetic are made up, the nerve- 

 components as they are termed, are capable of being classified in 

 accordance with the nature of the functions which they perform. 

 A broad division into motor and sensory fibres has already been 

 referred to. A more detailed classification is the following : 



Division I. Somatic sensory, comprising (a) the fibres which 

 have to do with general cutaneous or tactile sensations ; (ft) those 

 connected with the neuromast organs and with the auditory 

 organs ; (c) the fibres of the optic nerves. 



Division II. Visceral sensory, comprising (a) the fibres which 

 end in visceral mucosse and have to do with visceral sensations ; 

 (5) the fibres ending in taste-buds ; (c) those which terminate in 

 the olfactory epithelium. 



Division III Somatic motor, consisting of components which 

 terminate in somatic musculature. 



Division IV. Visceral motor, comprising all fibres which 

 terminate in visceral musculature and have to do with visceral 

 movements. 





