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MR. W. BEVAN LEWIS ON THE COMPARATIVE 
Section 2.—Central Projections op the Olpactory Organ. 
The Olfactory Lobe and Bulb. 
The olfactory lobe is constituted by a tubular prolongation of the cerebral cortex 
given off from the inferior aspect of the anterior pole of the hemisphere. Within the 
confines of the limbic fissure its cortex is directly continuous with that of both 
extremities of the limbic lobe along the lower and lateral aspects of the brain, whilst 
above and outside the boundaries of the limbic fissure it is overlapped by the projecting 
frontal extremity of the hemisphere, with the cortex of which its surface is also 
continuous. Its continuity with the anterior extremity of the upper and lower limbic 
arcs thus completes the circuit made by the great limbic lobe. 
The summit of the olfactory lobe is capped by the olfactory bulb, which completely 
ensheaths its extremity and receives the olfactory nerve fibres. 
The connexions o.f the olfactory cortex, inclusive of the bulb, are in this animal both 
complicated and extensive—complicated in that its medulla brings it into relation with 
most diverse regions of the brain, and extensive in so far that its fibres are projected 
back into the occipital pole of the hemisphere, and embrace in their circuit the whole 
extent of the limbic arc, thus associating the most distant realms of the brain in direct 
organic connexion. For convenience of description as well as upon anatomical 
grounds, we may consider the olfactory medulla as consisting of the following distinct 
systems :— 
1. A central decussating and commissural fasciculus. 
2. A connecting system with the striate body. 
3. An arciform series. 
4. A superficial medullated band. 
1. Central fasciculus .—A rapid convergence of fibres from the bulbus olfactorius 
constitutes the origin of a central medullated core for the olfactory lobe, which soon 
becoming almost cylindrical in contour courses backwards through this lobe, and 
arching slightly outwards plunges into the anterior extremity of the caudate nucleus. 
Owing to its outward curve, vertical sections through the frontal extremity exhibit an 
oval contour of this fasciculus, such oblique sections measuring 1'3 mm. in its greater 
diameter, by *78 mm. in its shorter diameter. It is, however, a uniformly cylindrical 
fasciculus, its usual diameter being - 87 mm. Receiving no fresh addition of fibres, the 
central olfactory fasciculi of both hemispheres attain their greatest distance apart 
within the corpus striatum (476 to 5-046 mm.), and thence converge towards the 
descending pillar of the fornix, passing in this course slightly upwards in relation to 
the cortex at the base, and lying betwixt the corpus striatum on the outer side and the 
thickened folia of the septum lucidum internally, in close relationship to the floor of 
the lateral ventricles. In the Rabbit, the central canal is seen lined with endothelium, 
