THE ELASMOBRANCH FISHES 



273 



(hr.) and anterior oblique semicircular (aus.) canals. The posterior utriculus 

 is the posterior connective of the posterior oblique semicircular canal. This 

 frequently, as in Heterodontus francisci {pos., fig. 236d), may be well devel- 

 oped, the two parts of the utriculus being widely separated by the sacculus and 

 endolymphatic duct. The posterior part of the utriculus has its connection 

 with the sacculus by an elliptical aperture (d.c.) in Squalus, but this connec- 

 tion is much longer in a type like Heterodontus. In Laemargus borealis the 

 posterior part of the utriculus is connected with 

 the sacculus by a long tube as it is also in Raja 

 clavata (d.c, fig. 237d). 



The semicircular canals, although assuming dif- 

 ferent degrees of compression, as is shown by a 

 comparison of the compact ear of Heptanchus 

 with the elongate ear of Squalus, are similarly ar- 

 ranged in three planes. One of these planes is an- 

 terior and oblique, another posterior and oblique, 

 and the third horizontal in position. The anterior 

 and horizontal canals join the utriculus proper, 

 pass forward and backward respectively and then 

 downward to their ampullae, which are in close 

 proximity. The posterior canal is similarly a con- 

 tinuation of the posterior part of the utriculus up- 

 ward and backward and downward to its ampulla 

 (figs. 236 and 237b and d). 



The ampullae (a.a., a.p., and a.h., figs. 236 and 

 237) are interesting from their terminal relations as end organs of the nerve. 

 A section through such an otic ampulla (fig. 238a) by Retzius (1881) shows 

 the crista acustica {cr.) which is the terminal mass of sense cells capped by 

 the cupula terminalis {cii.). A more detailed view cutting through the am- 

 pulla demonstrates two kinds of cells in the crista. One of these is the support- 

 ing or thread cell {t.c, fig. 238b) and the other is the sense or hair cell (h.c). 

 The latter of these projects into the endolymph of the ampullary cavity and is 

 capable of receiving sensations. Multitudes of cells of this sort are located in 

 all the ampullae and also in the sacculus, lagena, and macula neglecta. 



INNERVATION OF EAR 



The innervation of the ear is, as we have said for Heptanchus, through the 

 eighth or auditory nerve. Just before reaching the ear the nerve separates 

 into two main divisions (figs. 236 and 237). The anterior division, the vestib- 

 ular nerve, separates into an anterior ramus (ra.a.) to the anterior ampulla 

 and a second ramus to the ampulla of the horizontal canal (ra.h.). Other 

 branches from the stem supply the area of the recessus utriculi. The posterior 

 of these, the saccular nerve, finally reaches the ampulla (a.p.) of the posterior 

 canal. On its way it gives off a dorsal branch, the ramus neglectus (ra.n.) to 

 the macula neglecta of the sacculus, and ventral branches to the lagena and to 

 the sacculus (ra.s.). 



Fig. 239. Development of 

 the ear of Scyllium. (From 

 Krause.) 



aos., anterior oblique semi- 

 circular canal; e.d., endo- 

 lymphatic duct; hr., horizon- 

 tal canal; s., sacculus. 



