THE OLFACTORY TRACT AND BULB. 545 
surface, and, entering into the stratum zonale, divides into terminal filaments, which spread 
out in the tangential interlacement characteristic of this layer. 
Nerve Fibres. —The arrangement of the nerve-fibres can best be studied in vertical 
sections through the gray cortex, which have been specially treated with this end in 
view. In such preparations bundles of nerve-fibres are seen to radiate into the gray 
cortex from the surface of the white centre of the gyrus. As these proceed through the 
polymorphic layer into the layer of large pyramidal cells, they gradually become less 
distinct, and, finally, they disperse and are lost to view before they reach the layer of 
small pyramidal cells. In the intervals between the radiating bundles the polymorphic 
and large pyramidal cells are arranged in columns, and in the same intervals an open 
feltwork of intercrossing fibres is evident. After the radiating fibre-bundles have dis- 
appeared the same feltwork of fibres is visible in the gray matter, and consequently it is 
convenient to distinguish, with Edinger, an inter-radial feltwork and a supra-radial felt- 
work of fibres in the cortex. The fibres which enter into the composition of the different 
radial bundles vary in number from ten to twenty, and they gradually diminish in number 
as they proceed onwards. This diminution is due to their joining the various cells that 
they meet (both polymorphic and pyramidal) as their axons. The fibres in a given bundle 
also vary much in size, and it may be noticed that the largest fibres disappear in the 
vicinity of the large pyramidal cells, which shows clearly that it is with these that they 
are connected. But, in addition to cell-axons, the radial bundles contain fibres of an 
altogether different type, viz. corticipetal fibres, which pass through all the layers of the 
cortex and end in fine terminal filaments in the tangential interlacement of the stratum 
zonale, 
The inter-radial and supra-radial feltwork is largely formed of the collaterals which 
issue from the axons. By a condensation of this feltwork the two bands of Baillarger are 
formed. The outer band, which is the broader and better marked, occurs in the deeper 
part of the layer of the large pyramidal cells. The inner band, when present, is formed 
in the superficial part of the layer of polymorphic cells. 
Another condensation of the fibre-feltwork in the superficial part of the supra-radial 
region may be noted in certain localities. This is termed the band of Bechterew. 
It has been noted that up to a certain point the tangential fibres increase in quantity 
as age advances, and there is reason to believe that upon the richness with which the 
gray cortex is supplied with 
the tangential variety—depends to some extent the 
intellectual capacity of an individual. 
Whilst the general mass of the cortex for the most 
part conforms more or less closely to the ground-type 
described above, showing merely deviations characteristic 
of the different regions, there is one part of the cortex, [=< 
viz. the cornu ammonis and the fascia dentata, in which 
the structural arrangement of the elements is very | 
markedly different. To some extent this is due to the 
complicated manner in which, in this region, the cortex 
is folded upon itself. 
~ 
MITRAL CELL 
THe Ouractory Tracr AND BULB. 
“> LAYER 
The olfactory tract and bulb arise as a hollow out- oS 
growth from the primitive cerebral vesicle. In many aie 
animals with a well-developed olfactory apparatus, the 
tract and bulb remain hollow; but in man the central 
cavity becomes obliterated, although traces of the nee 
original hollow persist in the shape of ependymal J, PLATE 
remains, visible in the centre of both tract and bulb. 
Outside these ependymal elements is a coating of 
white matter, upon which is laid the gray matter. LITT LLL) MEMBRANE 
The gray matter, however, is by no means uniformly 
distributed over the surface. In the tract, except 
along the dorsal edge, it is so thinly spread that it is 
hardly appreciable. In the bulb, on the other hand, there is very little gray matter on 
the dorsum, but a considerable quantity on the ventral surface ; and it is into this that 
the delicate nerves which enter the cranium through the cribriform plate of the ethmoid 
bone sink. A brief description of the structure of this infrabulbar mass of gray matter, 
as well as of the connexions established by its elements, now becomes necessary. 
3D 
Fic. 406.—DIAGRAM OF THE MINUTE 
STRUCTURE OF THE OLFACTORY BULB. 
