STRUCTURE OF THE OLFACTORY TRACT AND BULB. 253 



one hand from the terminal arborisations of the non-medullated fibres 

 which form the subjacent layer, and on the other hand from arborisa- 

 tions of descending processes of the large " mitral " cells of the layer 

 above. 



4. This is the layer of olfactory nerve-fibres (fig. 284, 8), which are all 

 non-medullated, and are continued from the olfactory fibres of the 

 Scheiderian or olfactory mucous membrane of the nasal fossae. In 

 this mucous membrane they take origin from the bipolar olfactory 

 cells which are characteristic of the membrane (see Lesson XLIV., fig. 

 309), and they end in arborisations within the olfactory glomeruli, 

 where they come in contact with the arborisations of the mitral cells. 

 The relations of the olfactory cells and fibres to the mitral cells and 

 the continuation of the axis-cylinders of the latter upwards and back- 

 wards to join the optic tract are shown in the accompanying diagram 

 (fig. 285). These relations have only recently been elucidated by the 

 employment of the method of Golgi (see Appendix), chiefly by the 

 researches of Golgi himself and of R. y Cajal. 



Basal ganglia. Besides the grey matter of the cerebral cortex the 

 cerebral hemispheres conceal in their deeper parts certain other masses 

 of grey substance (figs. 286, 287). The principal of these are the 

 corpus, striatum (nucleus caudatus, n.c. and nucleus lenticularis, n.l.) and 

 optic thalamus (th.). Between them run the bundles of white fibres 

 which are passing upwards from the cms cerebri, forming a white 

 lamina termed the internal capsule. Above the level of these nuclei the 

 internal capsule expands into the medullary centre of the hemisphere. 



The nucleus caudatus of the corpus striatum is composed of a 

 reddish-grey neuroglia containing both moderately large and small 

 multipolar nerve-cells. It receives fibres from the part of the internal 

 capsule which separates it from the nucleus lenticularis, and next to the 

 lateral ventricle it is covered by a thin layer of neuroglia (ependyma), 

 and over this by the epithelium of the cavity. 



The nucleus lenticularis, which corresponds in position internally 

 with the island of Reil externally, is divided by two white laminas into 

 three zones. It is separated from the nucleus caudatus and optic 

 thalamus by the internal capsule (figs. 286, 287, i.e.), which consists of 

 the bundles of medullary fibres which are passing between the white 

 centre of the hemisphere and the crus cerebri ; it receives on its inner 

 side many white fibres from the capsule, and these impart to it a 

 radially striated aspect. Many of the nerve-cells of the nucleus lenti- 

 cularis contain yellow pigment. 



The optic thalamus, which lies at the side of the third ventricle 

 and forms part of the floor of the lateral ventricle, is covered externally 



