THE LATERAL VENTRICLES 873 



(o habenulo-peduncular tract), and fibres arising in this latter nucleus pass to the cells about the 

 central grey substance of the mesenecphalon (an inter-pedunculo-tegmental tract). The two 

 mesencephalic paths here noted and the mammillo-mesencephalic fasciculus noted above give 

 three anatomical possibilities for olfactory reflex activities, visceral (or sympathetic) and 

 somatic, involving the motor cranial nerves and possibly the spinal nerves. Fibres arising in the 

 cortex of the hippocampal g3Tus, uncus especially, may pass by way of the cingulum and thence 

 by any suitable association fasciculus of the cerebral hemisphere to the motor area of the cere- 

 bral cortex; also fibres may arise from the anterior nucleus of the thalamus and pass to the motor 

 cortex by way of the internal capsule. From the motor cortex, the descending pyramidal 

 fibres give the possibihties for any higher cortical activities induced by smeU. 



A more direct mesencephalic path has been suggested by Wallenberg, namely, that ceUs 

 in the olfactory trigone and anterior perforated substance, about which terminates fibres of 

 the ohactory tract, send axones directly posteriorly, around the tuber cinereum, to terminate 

 in the mammiUary body and thence the impulses may go to the mesencephalon. Such fibres, 

 if they exist, would form an olfacto-mammillary tract. A path is described in the hedge-hog 

 which arises from cells in the olfactory trigone and passes directly posteriorly to terminate in 

 the grey substance of the mesencephalon — an olfacto-mesencephalic tract. 



To the complicated central connections of the sense of smell, Dejerine adds yet another path, 

 namely, a portion at least of the terminal stria [stria terminalis] of the thalamus (taenia semi- 

 circularis). This contains fibres arising from cells in the anterior perforated substance and in 

 the septum pellucidum and fibres from the opposite side by way of the anterior commissure. It 

 runs a crescentic course posteriorly, bounding the thalamus from the caudate nucleus, turning 

 downward and then anteriorly in the wall of the inferior cornu of the lateral ventricle to termi- 

 nate in the amygdaloid nucleus, which latter is a more or less detached bit of the cortex of the 

 extreme anterior portion of the hippocampal gyrus (uncus). The stria is said also to contain 

 fibres which arise in the amygdaloid nucleus and course in it forward to be given off to the thala- 

 mus and probably to the internal capsule and thence to the cerebral cortex above. 



SUMMARY OF THE OLFACTORY APPARATUS 



I. Peripheral part. 



(1) Olfactory area of nasal epithelium containing the cell-bodies and peripheral processes 

 of olfactory neurones (olfactory ganglion). 



(2) Non-meduUated central processes of olfactory neurones, the olfactory nerve, passing as 

 numerous filaments through the cribriform plate of the ethmoid, to terminate in contact with 

 the dendrites of the "mitral cells" (stratum glomerulosum) in the olfactory bulb. 



II. The Rhinencephalon. 



A. The anterior division. 



(1) Olfactory bulb, olfactory tract, olfactory trigone (tubercle), lateral olfactory stria 

 (gyrus), medial and intermediate olfactory striae. 



(2) The parolfactory area, subcallosal gyrus, anterior perforated substance including the 

 diagonal band of Broca. 



B. The posterior division. 



(1) Part of anterior commissure, septum peUucidum, uncinate fasciculus, hippocampal 

 gyrus (uncus especially), dentate gyrus, gyrus cinguli and cingulum. 



(2) Fimbria, hippocampal commissure, fornix, longitudinal striae upon corpus caUosum, 

 mammiUary body, mammillo-thalamic fasciculus, mammiUo-mesencephahc fasciculus. 



(3) The anterior nucleus of the thalamus. 



(4) The medullary stria of the thalamus, habenular nucleus, fasciculus retroflexus, inter- 

 peduncular nucleus, and interpedunculo-tegmental tract. 



(5) Probably an olfacto-mammillary and an olfacto-mesencephalic tract, and a part of the 

 terminal stria of the thalamus with the amygdaloid nucleus. 



THE LATERAL VENTRICLES 



Two of the four cavities of the ventricular system of the brain are in the telen- 

 cephalon. From their position, one in each cerebral hemisphere, they are known 

 as the lateral ventricles. They arise as lateral dilations of the cavity of the anter- 

 ior of the primary vesicles, and, just as the fourth ventricle remains in communi- 

 • cation with the third by way of the aqueduct of the cerebrum, so the lateral are 

 connected ^dth the third by the two interventricular foramina (Monroi). The 

 whole ventricular system, including the central canal of the spinal cord, is hned 

 by a continuous layer of ependyma and contains a small quantity of liquid known 

 as the cerebro-spinal fluid. 



Each lateral ventricle is of an irregular, horseshoe shape. It consists of a 

 central portion or body and three cornua, which correspond to the three poles of 

 the hemisphere. The portion projecting into the frontal lobe is known as the 

 anterior cornu, that projecting into the occipital lobe is the posterior cornu, and 

 the portion which sweeps anteriorly downward into the temporal lobe is the 

 inferior cornu. The ventricles of different individuals vary considerably in capac- 

 ity, and the cavity of a given ventricle is not uniform throughout. In some 



