olfactory nerve and nasal organ. 239 



Section ii. — The Olfactory Nerve and Nasal Organ. 



The olfactory bulbs are in part overshadowed by the 

 cerebrum. The bulb gradually tapers into the cylindrical 

 olfactory nerve, which penetrates the membranous wall of 

 the cranium and then continues cephalad along the lateral 

 face of the internasal cartilage. As the latter begins to 

 expand around the nasal sac, the nerve is crowded under 

 the m. obliquus superior near its origin and then pene- 

 trates the internasal cartilage through a foramen which is 

 in part lined by a V-shaped projection from the pareth- 

 moid bone. Having reached the olfactory fossa, it runs 

 along the inner side of the non-sensory part of the olfac- 

 tory sac, being separated from it, however, by a great 

 lymph sinus. Here it breaks up into numerous branches 

 which enter the lamellae of the sensory portion of the sac. 



The nasal organs of Menidia are large and well devel- 

 oped, like the other organs of special sense. The anterior 

 (cephalic) opening is a very small pore {n. a. «.), lying 

 laterally of and close to the cephalic tip of the supra- 

 orbital canal. This pore opens directly into the cephalic 

 end of a wide sac containing three large lamellae, which 

 are attached to the ventro-mesal wall of the sac and which 

 extend lengthwise for almost the entire length of the 

 latter, and two smaller lamellae at the cephalic end of the 

 sac. These lamellae and the walls of the sac adjacent (but 

 not the dorso-lateral wall of the sac) are thickly covered 

 with the bud-like groups of sensory cells so characteristic 

 of the teleosts (Blaue, '84). 



These "olfactory buds" are closely packed in the 

 mucous membrane along the whole surfaces of the lamellae 

 except at their tips, very much like Blaue's figure of 

 Trigla, though not with so great regularity as his draw- 

 ings would indicate. The "olfactory buds" vary in size, 



