1086 OLFACTORY SENSATIONS. [BOOK m. 



fine medullated kind; it also contains, in no large number in 

 man, nerve cells, some of which from their triangular form and 

 tapering branched processes are not unlike the pyramidal cells of 

 the cortex. The larger of these cells are generally found near 

 the nuclear layer. Next to this molecular layer, or 'gelatinous 

 layer' as it is sometimes called, comes, still working outwards 

 towards the surface, a characteristic layer in which are found the 

 'olfactory glomeruli'; and outside this is the layer of olfactory 

 fibres proper, that is to say, fibres non- medullated (70) but 

 bearing an obvious neurilemma. These olfactory fibres are 

 arranged in a close set plexus, and bundles of fibres gathered 

 up from the plexus at intervals pierce the pia mater, which 

 invests the bulb and furnishes it with an ample supply of blood 

 vessels, to form the olfactory nerve proper. The structure of the 

 olfactory glomeruli, which are about '05 mm. in diameter, has 

 not yet been fully made out; they are described as being formed 

 by coils of the olfactory fibres with small cells and blood vessels 

 interspersed among the coils ; in the lower animals a finely granu- 

 lar ground substance is present. Fibres from the layers beneath 

 have been traced to them. We may perhaps assume that they 

 serve as the immediate origin of the olfactory fibres ; but their 

 exact relations to the other layers of the bulb are by no means 

 clear. 



The tract is composed partly of longitudinal fibres, with which 

 are mingled nerve cells, and partly of neuroglial gelatinous substance. 

 The fibres begin in the bulb, which appears to serve as a relay 

 between them and the fibres of the olfactory nerve proper; and 

 while some appear to end in cells in the tract itself, others are 

 continued on to the end of the tract, being joined by fibres taking 

 origin along the tract. We may compare the bulb and the tract 

 to a part of the retina (as we shall see, a part of the retina 

 corresponds to the olfactory mucous membrane) and the optic 

 nerve. 



The dorsal surface of the tract is adherent to and continuous 

 with the substance of the cerebral hemisphere, in a groove of 

 which it lies, but the tract may be considered as independent 

 of the hemisphere until it reaches its end, at which it breaks 

 up into bands of fibres, spoken of as its 'roots.' The most 

 conspicuous of these is a lateral one, which sweeping laterally 

 across the anterior perforated space, at the mouth of the fissure of 

 Sylvius, may be traced to the nucleus amygdalae (Fig. 116, Na), 

 and the junction of this with the hippocampal or uncinate gyrus 

 (Fig. 130) in the temporal lobe of the hemisphere of the same 

 side. A much smaller median one, which however in some of 

 the lower animals is large and conspicuous, takes a median 

 direction, passes into the anterior commissure (635) and so 

 reaches the olfactory tract of the opposite side. Other small 

 roots have also been described. 



