112 



ZOOLOGY 



SECT. 



ass 



As the membranous labyrinth develops in the embryo, it becomes 

 surrounded*and 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 perilymph, 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 



FIG. 798 External view of organ of hear- ma 7 ^ e regarded as a highly 



specialised part. 



Nerve - components. The 



nerve-fibres of which the nerves - 



circular canal ; cus. canal uniting sacculus rprphral Qrvinnl anrl \rmrafVifi^ 

 with utriculus ; de. endolymphatic duct ; Dral > S P mal > ancl 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 ; (6) 

 th o s e connected 

 with the neuro- 

 mast organs and 

 with the auditory 



(c) the FIG. 799. Longitudinal section through an ampulla. a. c. 

 f } ' auditory epithelium ; a. h. auditory hairs ; c. part of somi- 



OI tile ODtlC circular canal ; cr. crista acustica ; ct. connective-tissue ; e, i. 



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

 Foster and Shore's Physiology.) 



r.e 



ing of Craniata (semi-diagrammatic) 

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

 zontal canal ; ap. of posterior canal ; ass. 

 apex of superior utricnlar sinus ; ca. an- 

 terior, ce. horizontal, cp. posterior semi- 

 circular canal ; cus. canal uniting sacculu 



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

 lus ; se. endolymphatic sac ; sp. posterior 

 utricular sinus ; ss. superior utricular 

 sinus ; tt. utriculus. (From Wieders- 

 heim's Vertebrata.) 



