THE OLFACTORY APPARATUS. 1 77 



cells, the remainder act as supporting cells. The sense cells 

 differ from those in any other sense organs of vertebrates and 

 resemble a common type of sense cells in invertebrates. Each is 

 a slender spindle-shaped cell from the inner end of vi^hich a nerve 

 fiber arises and runs toward the brain (Fig. 93). The fibers 

 grow out from the olfactory cells during development and enter 

 the olfactory bulbs of the forebrain, where they end in a manner 

 to be described below. These fibers constitute the olfactory 

 nerve. They never have myelin sheaths. 



The olfactory centers. In fishes the greater part of the 

 forebrain is related exclusively to olfactory impulses. The primary 



Fig. 93. — A section of the olfactory epithelium of a bony fish at the time of 

 hatching, to show the origin of the fibers of the olfactory nerve. 



center for olfactory fibers is the olfactory bulb. In cyclostomes the 

 bulb consists of several layers of cells surrounding the ventricle 

 and of a superficial zone of fibers (Fig. 94). Nearly all the cells 

 are either spindle-shaped and provided with a single dendrite 

 vertically placed in the wall of the bulb or are stellate and have 

 two or more dendrites which run obliquely toward the surface. 

 The final branches of the dendrites are intermingled with the 

 end-branches of olfactory fibers near the surface of the bulb. 

 The olfactory fibers run in bundles as they enter the bulb and as 

 the individual fibers divide and subdivide the bundles form larger 

 or smaller cylindrical masses of interwoven nerve fibrils. Into 

 each of these masses the dendrites of numerous cells penetrate 

 and so receive impulses from the olfactory fibers. The combined 

 mass of fibers and dendrites is a glomerulus. There is no observable 

 difference in the relations of the various nerve cells of different 

 sizes and shapes to these glomeruli. There is to be seen, how- 

 ever, the beginning of a special type of cells which come to pre- 



