112 



ZOOLOGY 



SECT. 



se 



ca 



ass 



As the membranous labyrinth develops in the embryo, it becomes 

 surroundecTand enclosed by the auditory capsule, the cartilage of 



which adapts itself to the form 

 of the labyrinth, presenting a 

 large excavation for the utricle 

 and saccule and tunnel-like pas- 

 sages for the canals. The audi- 

 tory organ does not, however, fit 

 tightly into this system of cavi- 

 ties, but between it and the 

 cartilage is a space, filled by a 

 fluid called peritymph, which acts 

 as a buffer to the delicate organ 

 floating in it. 



The early history of the audi- 

 tory 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-com- 

 ponents as they are termed, are 

 capable of being classified in ac- 

 cordance 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 classi- 

 fication is the 

 following : 



Division I. 

 Somatic sensory, 

 comprising (a) the 

 fibres which have 

 to do with general 

 cutaneous or tac- 

 tile sensations ; (b) 

 th o s e connected 

 with the neuro- 

 mast organs and 

 with the auditory 



orffailS (c\ the Jl ' la - 799. Longitudinal section through an ampulla. a. c. 



' , v ' auditory epithelium ; a. h. auditory hairs ; P. part of seini- 



fibres Ol the OptlC circular canal; cr. crista acustica ; ct. connective-tissue; e,i. 



epithelium ; n. nerve ; v. junction with utriculus. (From 

 nerves. Foster and Shore's Physiology.) 



FIG. 708. External view of organ of hear- 

 ing of Craniata (semi-diagrammatic). 

 an. ampulla of anterior canal ; ae. of hori- 

 zontal canal ; ap. of posterior canal ; ass. 

 apex of superior utricular sinus ; ca. an- 

 terior, ce. horizontal, cp. posterior semi- 

 circular canal ; CMS. canal uniting sacculus 

 with utriculus ; de. endolymphatic duct : 

 7. cochlea ; rec. utricular recess ; s. saccu- 

 lus ; se. endolymphatic sac ; sp. posterior 

 utricular sinus ; s,s. superior utricular 

 sinus ; u. utriculus. (From Wieders- 

 heim's Vertebra ta.) 



