214 



ZOOLOGY 



SECT. 



leaving the brain, the right nerve going to the left, and the left 

 nerve to the right eye. 



Sensory Organs. — The most distinctive feature of the olfactory 

 sac is the possession of two small apertures, the anterior provided 

 with a valve. 



The eye (Fig. 867) has a very flat coo-nea {en.) with which the 

 globular lens (I.) is almost in contact, so that the aqueous chamber 

 of the eye is extremely small. Between the cartilaginous sclerotic 

 (scl.) and the vascular choroid (ch.) is a silvery layer or argentea 

 (arg.) which owes its colour to minute crystals in the cells of 

 which it is composed. In the posterior part of the eye, between 

 the choroid and the argentea, is a thickened ring-shaped structure 

 (ch. gld.) surrounding the optic nerve, and called the choroid gland : 



it is not glandular, but is 



scl- cp.JvoLL 



c-n 



xr 



ch.^Zd, 



opt.TW 



h^ld. 



PS 



a complex network of 

 blood-vessels, or rete mira- 

 bile. It is supplied with 

 blood by the efferent artery 

 of the pseudobranch. Close 

 to the entrance of the optic 

 nerve a vascular fold of the 

 choroid, the falciform pro- 

 cess {pr. gl.) pierces the 

 retina, and is continued to 

 the back of the lens where 

 it ends a knob, the cam- 

 panula Halleri (cp. hal.), 

 which contains smooth 

 muscular fibres. The falci- 

 form process with the cam- 

 panula Halleri take an im- 

 portant part in the process 

 of aecomniodation by which 

 the eye becomes adapted to forming and receiving images of 

 objects at various distances. Accommodation ia the Fish is 

 effected, not by an alteration in the curvature of the lens as in 

 higher Vertebrates, but by changes in its position, by which it 

 becomes more approximated towards, or further withdrawn from, 

 the retina. In bringing about these changes of position, the 

 structures in question appear to play the principal part. 



The auditory organ (Fig. 868) is chiefly remarkable for the large 

 size of the otoliths (ot. 1 — 3). They are three in number; one, 

 called the sagitta (st. 1), is fully 6 mm. in length, and almost fills 

 the sacculus : another, the asteriscus (ot. 2), is a small granule lying 

 in the lagena or rudimentary cochlea : the third, the lapilkis (ot. 3), 

 is placed in the utriculus close to the ampullae of the anterior and 

 horizontal canals. 



Fig. S67.— Salmo fario. Vertical section of eye 

 (semi-diagrammatic), arg. argentea ; ch. choroid ; 

 ch. (/Id. choroid gland ; en. cornea ; cp. hL cam- 

 panula Halleri ; ir. iris ; I. lens ; opt. n. optic 

 nerve ; pg. pigmentary layer ; pr. ft., processus 

 falciformis ; scl. sclerotic (dotted). 



