Rhixexcephalox 89 



All these elements in the olfactory bulb are arranged in 

 definite layers (PI. XXW). Superficially, we find, as would 

 be expected, a thin layer of unmyelinated fibres made up of 

 the entering primary olfactory axons {stratum fibrorum , lamina 

 fibronim ntrvi olfactorii). This passes into a layer in which 

 the glomeruli are seen lying side by side in a single series 

 {lamina glomeriilosa) . A thin band of granule cells {lamina 

 granulosa externa) lies just inside this, and a stratum made 

 up of dendrites and unmyelinated fibres {lamina gelatinosa) 

 separates the latter from the perikarya of the mitral cells, 

 which are disposed in a single row {lamina cellular um). 

 Alternating layers of granule cells and unmyelinated fibres 

 w4th a few fine myelinated fibres {lamina granulosa interna) 

 lie inside the layer of mitral cells, and between the former and 

 the centre of the bulb are two layers of myelinated fibres 

 {lami7ta fibrorum tractus olfactorii and lamina fibrorum medullae 

 lobi piriformis). The fibres in the outer of the two last 

 mentioned layers are grouped in little bundles which run 

 together posteriorly to form the olfactory tract. Those in 

 the innermost layer have thinner myelin sheaths, are scattered 

 in a rather diffuse reticulum, and run into the deep white 

 matter of the lateral olfactory gyrus (vide infra). They 

 include the fibres to and from the anterior commissure (p. 97). 

 Among them are many small cells, which are most numerous 

 near the centre, where the ventricle is situated. 



Some of the olfactory nerve fibres (the vomeronasal nerve) 

 separate from the rest and run to a small body which is 

 imbedded in the posterior part of the dorsal surface of the 

 olfactory bulb. This is the accessory olfactory bulb or bulbus 

 parolfactorius (PL XXV.). Its structure is similar to that 

 of the olfactory bulb, except tbat the internal granular layer 

 is nearly absent and the fibres to the olfactory tract lie between 

 the mitral layer and the internal granular layer of the olfactory 

 bulb. These fibres run in the lateral olfactory stria to the 

 amygdaloid complex (p. 92). 



