420 PHYSIOLOGY 



laris and nucleus caudatus of the corpus striatum. Outside the corpus striatum we find 

 another mass of white fibres, known as the external capsule, arid this is separated from 

 the white matter of the cortex cerebri by a thin layer of grey matter known as the claus- 

 tnun. 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 1 1n- 

 optic thalamus from the lenticular nucleus. The part where the two limbs romp 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 I ho 

 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. 416). 



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

 actually on the surface of the body. These are 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 il 

 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 a> 

 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 back- 

 wards 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 grry 

 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 unci- 

 nate gyms, the subcallosal gyrus, the septum lucidum, and the hippocampal con volut ion. 

 The two sides of the rhinencephalon are united by fibres passing through the anterior 

 commissure. Other tracts subserving this apparatus include the hahenula passing 

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

 this to the interpeduncular ganglion, and the corpus mammillare which is connecteo 1 

 with the column of the fornix on the one hand and through the bundle of Vicq d'A/\ r 

 with the thalamus on the other. 



THE CHIEF TRACTS OF THE CEREBRAL HEMISPHERES 



We may divide the tracts of the upper brain or cerebral hemispheres into 

 three classes : 



I. Tracts connecting the brain with lower levels of the central nervous 

 system. 



