X SENSE-ORGANS 517 



taste-buds, are present on the papilla of the tongue (p. 492) 

 and on the soft palate (compare pp. 179 and 180). 



The organs of smell are situated in the olfactory capsules, 

 the form of which has already been described (p. 476). 

 They open externally by the external nostrils, and are pro- 

 duced backwards above the palate into the passage of the 

 internal nostrils, which communicate with the naso-pharynx 

 (Fig. IZ5 /. n, n.ph, p. 495). The olfactory epithelium, 

 supplied By the olfactory nerves, is situated on the ethmo- 

 turbinal (e. tb) : the mucous membrane of the maxillo-tur- 

 binal (in. tb) probably serves merely to warm the inspired air. 



On ihe ventral side of the nasal septum is a pair 01 small, tubular 

 structures known s& Jacobsoii s organs (Fig. 125, j), lined by epithelium 

 and enclosed in cartilages situated just to the inner side of the palatine 

 processes of the premaxillee (p. 477). Each of them opens anteriorly 

 into the corresponding naso-palatine canal (p. 490), and receives a 

 special branch of the olfactory nefve. The function of these organs is 

 not understood. 



The' Structure of the ^^^(Fig. 57) is similar to that already 

 described in other Vertebrates (pp. 1 8 r and 449), except that 

 the sclerotic is not cartilaginous, but is composed of dense 

 fibrous tissue, and the lens is relatively smaller than in the 

 dogfish and frog and is markedly biconvex in form, the outer 

 surface being rather flatter than the inner : it is capable of 

 adjustment by means of the ciliary inuscles contained in the 

 radiating ciliary processes (C.F) into which the choroid is 

 thrown just externally to the iris (compare p. 184). 



The eyelids have already been described on p. 469. The four recti 

 muscles ensheath the optic nerve, as in the frog (p. 186, compare Fig. 

 117), but \!a.& superior oblique, 'vasX.^?i<X of arising— ^like 'Casinferior obliqiie 

 — in the aiiterior part of the orbit, takes fts origin further back, neaf the 

 recti, passes forwards through a fibro-cartilaginous pulley at the anterior 

 angle of the orbit, and then backwards and outwards to its insertion on 

 the eyeball. 



