STRUCTURAL ARRANGEMENTS OF CEREBRUM 421 



masses of nervous material constituting the corpus striatum. These present 

 two nuclei of grey matter, known as the nucleus caudatus and the nucleus 

 lenticularis (Fig. 212). The crusta of the crura cerebri as it ascends to the 

 cerebral hemispheres passes behind between the optic thalamus and the 

 corpus striatum, and in front between the nucleus lenticularis and nucleus 

 caudatus of the corpus striatum. Outside the corpus striatum we find 

 another mass of white fibres, known as the external capsule, and this is 

 separated from the white matter of the cortex cerebri by a thin layer of grey 

 matter known as the claustrum. In a horizontal section through the 

 brain, the part of the internal capsule which pierces the corpus striatum 

 forms an angle with the posterior part separating the optic thalamus from 

 the lenticular nucleus. The part where the two limbs come in contact is 

 known as the genu of the internal capsule (Fig. 212). 



THE OLFACTORY APPARATUS OF THE BRAIN 

 In man the olfactory sense is but feebly developed, and the parts of the 

 brain connected therewith are inconspicuous in comparison with those en- 

 gaged in the reception of impressions from the other two main projicient 

 sense- organs^ namely, sight and hearing. On this account it is not easy to 

 make out the connections of the olfactory lobe proper, the rhinencephalon, 

 with the primitive part of the cortex, the archipallium, subserving the olfac- 

 tory sense and probably the allied sensations derived from the mouth cavity. 

 The wide connections of the olfactory sense-organs with the different parts 

 of the brain in the lower vertebrate are shown in the diagrammatic figure of 

 the brain of a reptile (Fig. 208, p 417). 



It is interesting to note that the olfactory nerve fibres are derived from cells situated 

 actually on the surface of the body. These aie bilateral, spindle-shaped cells, lying 

 in the olfactory mucous membrane at the upper part of the nasal cavity. The peri- 

 pheral process is short and passes towards the surface, while the deep process passes 

 as a non-medullated nerve-fibre through the cribriform plate of the ethmoid to sink 

 into the olfactory bulb. The bulb, in man, is a greyish enlargement at the anterior 

 end of the olfactory tract. In sections stained by Golgi's method of impregnation it 

 may be seen that the olfactory fibres terminate in an arborisation in close connection 

 with a thick end arborisation derived from a dendrite of a large nerve-cell, known as 

 a mitral cell. The synapses between these two sets of fibres are prominent objects 

 in a section through the olfactory bulb and form the ' olfactory glomeruli ' (Fig. 213). 

 The axons of the mitral cells pass back in the olfactory tracts. Each olfactory tract 

 divides posteriorly into two roots, the mesial root which curves inwards behind Broca's 

 area and passes into the end of the callosal gyrus, and the lateral root which runs 

 backwards and over the outer part of the anterior perforated spot. Its fibres pass 

 into the uncinate extremity of the hippocampal gyrus. The small triangular field of 

 grey matter between the diverging roots of the olfactory tract is known as the olfactory 

 tubercle. The primitive rhinencephalon includes in the adult human brain the 

 olfactory bulb and tract, together with the anterior perforated space, the anterior 

 part of the uncinate gyrus, the subcallosal gyrus, the septum lucldum, and the hippo- 

 campal convolution. The two sides of the rhinencephalon are united by fibres passing 

 through the anterior commissure. Other tracts subserving this apparatus include 

 the habenula passing from the fornix to the ganglion of the habenula, the fasciculus 

 retroflexus passing from this to the interpeduncular ganglion, and the corpus maimmllare 

 which is connected with the column of the fornix on the one hand and through the bundle 

 of Vicq d'Azyr with the thalamus on the other. 



