190 



4.— The Ear (Fig. 24). 



The aiiditoiy nerve is described with the other 

 cranial nerves. 



As in Teleosts generally the ear is only externally 

 enclosed by the ear ossicles. Hence it projects freely into 

 the skull cavity internally, and is only separated from the 

 brain by its own and the brain membranes. 



The utricular sac consists of a central chamber [utr., 

 often divided into three portions, two of which constitute 

 the utriculus sensu stricto and the third the posterior 

 utricular sinus), and a wide ventral chamber or superior 

 utricular sinus or canal commissure [utr.^). These, how- 

 ever, may conveniently be called the central and vertical 

 chambers of the utriculus. From the anterior end of the 

 central chamber connections are effected with the ampullae 

 of the anterior and external semicircular canals, at the 

 posterior end with the ampulla of the posterior semi- 

 circular canal and the other extremity of the external 

 canal. The vertical chamber rises up almost at right- 

 angles from about the centre of the central chamber, and 

 receives above the upper extremities of the anterior and 

 posterior semicircular canals. There is only one sense 

 organ in the utriculus, the macula acustica recessus 

 utriculi [m. r. u.), situated in a slight depression of the 

 central chamber in front (E-ecessus utriculi). The three 

 semicircular canals are disposed as follows: — 



Ganalis anterior (a. *•. c). — -Just above its connection 

 with the central chamber of the utriculus it swells into a 

 large ampulla anterior [amp. a.), the outer wall of which 

 bears a transversely extending sense organ and ridge, the 

 crista acustica ampullae anterioris {ant. cr.). Above, the 

 canal passes upwards and backwards into the vertical 

 utricular chamber. 



