XIII 



PHYLUM CHORD ATA 



199 



FIG. 819. Salmo fario. Vertical section of eye 

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

 ch. gld. choroid gland ; c/i. cornea ; cp. hal. cam- 

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

 nerve ; pg. pigmentary layer ; pr.jt. processus 

 falciformis ; scl. sclerotic (dotted). 



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. 819) has a 

 very flat cornea (en.) with 

 which the globular lens (I.) 

 is almost in contact, so that 

 the anterior chamber of 

 the eye is extremely small. 

 Between the cartilaginous 

 sclerotic (scl.) and the vas- 

 cular choroid (ch.) is a sil- 

 very 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 thick- 

 ened ring-shaped structure 

 (ch. gld.) surrounding the 



optic nerve, and called the choroid gland: it is not glandular, 



but is a complex network of blood vessels or rete mirabile. It is 



supplied with blood by the 



efferent artery of the 



pseudo branch. Close to the 



entrance of the optic nerve a 



vascular fold of the choroid, 



the falciform process (pr. gl. ), 



pierces the retina, and is 



continued to the back of 



the lens where it ends a 



knob, the campanula Hal- 



leri (cp. hal.), which contains 



smooth muscular fibres, and 



is probably concerned in 



accommodation by altering 



the curvature of the lens. 

 The auditory organ (Fig. 



820) is chiefly remarkable 



for the large size of the 



otoliths (ot. 1-3). They are 



three in number : one, 



called the sagitta (ot. 1), is 



fully 6mm. in length, and 



almost fills the sacculus : another, the astcriscus (ot. 2), is a small 



granule lying in the lagena or rudimentary cochlea : the third (ot.3) 



cc.s.c 



FIG. 820. Salmo fario. The right auditory organ, 

 from the inner side ; the otoliths are shown 

 separately below. a. s. c. anterior semicircular 

 canal ; aud. nr. auditory nerve ; h. s. c. horizontal 

 canal ; ot. 1 3, otoliths ; p. s. c. posterior canal ; 

 sac. sacculus ; ut. utriculus. 



