34 



HIGH SCHOOL ZOOLOGY. 



41. Olfactory Organ. — Wlien the i-oof of the nasal sac 

 is removed (§ 4), the flooi- will be seen to be formed of a reddish 

 mucous membrane, presenting a median groove and a series of 

 transvei-se ridges running towards it. A current is established 

 from one nostril to the other, and the odoriferous particles con- 

 tained, are detected by special olfactoiy cells, which are situated 

 between the ridges, and are directly connected with the olfac- 

 tory nerve-fibres. In some fishes the ridges ai'e arranged in 

 such a way ;is to suggest to anatomists that the nasal sacs are 

 altered gills, which have been confined to this sensitive func- 

 tions, ^ but the examination of the catfish alone would not 

 suggest this view. 



42. The Eye. — In higher Vertebrates the eye affects the 

 shajie of the skull considerably moi-e than it does in the catfish, 

 for there is no orbit in the latter, and the eye is simply situated 

 in some fatty tissue between the overhanging frontal bone, and 

 the great muscle of the jaw. It is small in size and unprotected 

 by lids, the skin being thin and transpai-ent where it passes over 



the surface of the bulb, and 

 sufficiently loose to allow 

 the latter some independent 

 movement. This is effected 

 by six muscles, four of 

 which, the straight muscles 

 or Recti, are grouped above, 

 below and on either side of 

 the optic nerve, as it courses 

 from the optic foramen to 

 the bulb, and two others, the 

 oblique muscles, cross trans- 

 versely to the eye from the 

 skull. All the muscles are at- 



Fig. 13.— Vertical Section of Eye of Catfish. X 30 



S, skin; co. cornea; sc, sclerotic; ch, choroid; tacJied near the equator 01 



r, retina; o. entrance of the optic nerve; i, iris; xi,„i .,11 j. 4.1 , „«l„_„4.j- j. 



1, lens; vh, vitreous humor. thebulbto theSClerotlCCOat. 



