106 



ZOOLOGY 



SECT. 



sac, supplied, however, by paired olfactory nerves. The sac is 

 lined by the olfactory mucous membrane or Schneiderian membrane, 

 the epithelium of which contains peculiar, elongated sensory cells 

 (Fig. 792), their free ends often produced into hair-like processes. 

 In the Dipnoi and all higher groups the posterior end of each sac 

 communicates with the cavity of the mouth by an aperture called 

 the posterior nostril, and an analogous communication occurs in 

 the case of the unpaired organ of the Hags (vide p. 135). 



In many air-breathing Vertebrates there is formed an offshoot 

 from the olfactory organ, which, becoming separated, forms a 

 distinct sac lined with olfactory epithelium and opening into the 



ex. f- 



G.R 



C.CK, 



Civ 



FIG. 792. Epithelial cells of 

 olfactory mucous membrane. 

 A, of Lamprey ; B, of 

 Salamander. E. inter- 

 stitial cells ; R. olfactory 

 cells. (From Wiedersheim's 

 Vertebrala.) 



FIG. 793. Diagrammatic horizontal section of the eye of 

 Man. c. cornea ; Ch. choroid (dotted) ; C. P. ciliary 

 processes : e. c. epithelium of cornea ; e. cj. conjunctiva ; 

 /. o. yellow spot ; /. iris ; L. lens ; O.N, optic nerve ; os. 

 ora serrata ; o x. optic axis ; p.c.R. anterior non-visual 

 portion of retina ; P. E. pigmented epithelium (black) ; 

 R. retina ; sp. 1. suspensory ligament ; Scl. sclerotic ; V. H. 

 vitreous chamber. (From Foster and Shore's Physiology.) 



mouth. This is Jacobson's organ : it is supplied by the olfactory 

 and trigeminal nerves. 



The paired eye is a more or less globular structure, lying in 

 the orbit, and covered externally by a thick coat of cartilage or of 

 dense fibrous tissue, the optic capsule or sclerotic (Fig. 793, scl.). 

 On the outer or exposed portion of the eye the sclerotic is replaced 

 by a transparent membrane, the cornea (c), formed of a peculiar 

 variety of connective-tissue, and covered on both its outer and 

 inner faces by a layer of epithelium. The whole external coat of 

 the eye has thus the character of an opaque spherical case the 

 sclerotic, having a circular hole cut in one side of it and fitted with 

 a transparent window, the cornea. The curvature of the cornea is 



