278 



CHEMICAL SENSES 



Below the basal lamina the axons of the bipolar olfactory cells aggregate 

 in bundles of various size. In fishes, these fila olfactoria join and form 

 fasciculi, which pass through the base of the epithelial folds, or lamellae. 

 Increasingly larger trunks are formed; in some species, these together con- 

 stitute the olfactory nerve, which is wrapped in the cytoplasm of Schwann's 

 cells and is made up of a great many extremely thin fibers. The nerve passes 

 through the cribriform plate into the cranial cavity. However, in selachians 

 and in a number of other fishes no single olfactory nerve is formed, and the 

 numerous fila continue directly to the olfactory bulb, close to the olfactory 

 sac. The fibers spread over the surface and into the bulb, where they synapse 

 with mitral cells or tufted cells in the olfactory glomeruli. 



The basic structure of the olfactory bulb is similar in all vertebrates from 

 cyclostomes to primates. An early study of this structure in Mustelus was 

 made by Asai (1913). According to this author, the glomeruli in this species 

 measure 0.455-0.050 mm in size and can be irregularly distributed in the 

 mitral layer. In some areas they are arranged in two or three rows. Andres 

 (1970) distinguishes five histological layers: (a) olfactory nerve layer, (b) 

 glomerular layer, (c) mitral cell layer, (d) plexiform layer, and (e) periven- 

 tricular layer (Figure 7). (In terrestrial vertebrates an additional internal 

 plexiform layer can be defined.) He does not recognize the "granule cell 

 layer," frequently referred to in the literature. In Scyliorhinus canicula the 

 mitral cells have particularly thick dendrites (Figures 8, 9) and the plexi- 

 form layer is very rich in granule cells. 



000 u(500 000 no 

 oo op oooA) 



Figure 7 Fibers and cytoarchitecture of vertebrate olfactory bulbs: olfactory 

 nerve layer (SN), glomerular layer (SG), mitral cell layer (M), plexiform layer 

 (PL), periventricular layer (VL), and external plexiform layer (IPL). (a) Lamprey, 

 (b) elasmobranch, (c) amphibian, (d) reptile, (e) mammal. From Andres (1970). 



