536 THE CEREBRUM. 



which communicates, through that of the infundibukim, with the third ventricle. 

 This body is constantly present, and has the same connection with the brain in 

 all vertebrate animals. 



The optic commissure, or cldasma (fig. 375j, is constituted by the 

 union of the two optic tracts in front of the tnber cinerenm, and from it 

 the two optic nerves proceed. In it the nerve fibres of each side undergo 

 a partial decussation. The Larger proportion of the fibres of each optic 

 tract cross over to the opposite side, a smaller proportion proceed to 

 the eye on the same side, and other fibres are said to pass from one 

 optic tract to the other along the posterior portion of the comriissure, 

 while others pass from one nerve to the other along the anterior part of 

 the commissure. Thus each angle is rounded by fibres which pass 

 between its adjacent limbs. The optic tracts are said to receive an 

 accession of fibres from the lamina cinerea, and also from the " basal 

 optic ganglion," a collection of grey matter which Jies on the outsr side 

 of the tuber cinereum, and is the source of fibres which pass immediately 

 without decussation, to the optic nerve of the same side. 



In the middle line of the base of the brain, in front of the optic 

 commissure, is the anterior portion of the great longitudinal fissure, 

 which passes down between the hemispheres. At a short distance in 

 front of the commissure, this fissure is crossed transversely by a white 

 mass, which is the anterior recurved extremity of the corpus callosum. 

 On gently turning back the optic commissure, the lamina cimrca is 

 seen, a thin connecting layer of grey substance, occupying the space 

 between the corpus callosum and the commissure, and continuous above 

 the commissure with the tuber cinereum. It is connected at the sides 

 with the grey substance of the anterior perforated space, and forms 

 part of the anterior boundary of the third ventricle : it is somewhat 

 liable to be torn in removing the brain fi'om the skull ; and, in that 

 case, an aperture would be made into the fore part of the third 

 ventricle. 



At a short distance outwards from the lamina cinerea is the 

 anterior perforated space (locus pcrforatus anticus), a depression 

 near the entrance of the Sylvian fissure, floored with grey matter, and 

 pierced with a multitude of small holes for the passage of blood-vessels, 

 most of which are destined for the corpus striatum, — the adjacent por- 

 tion of the brain beneath which it lies. 



The grey surface of each perforated space is crossed by a broad white 

 band, which may be traced from the middle of the under surface of the 

 corpus callosum in front, backwards and outwards along the side of 

 the lamina cinerea towards the entrance of the Sylvian fissure. These 

 bands of the two sides are named the jjedancks of tlie corpus callosum. 



Olfactory tract and bulb (figs. 375, 1, and 376, 1'). — From the 

 front of the anterior perforated spot a nerve-like process extends — the 

 olfactory tract. It is lodged in a hollow (olfactory sulcus) in the under 

 (orbital) surface of the frontal lobe, close to the longitudinal fissure. 

 it ends anteriorly in an oval swelling — the olfactory bulb, which con- 

 sists chiefly of grey substance, and gives origin to the small nerves 

 which proceed, through the foramina of the ethmoid bone, to the nose. 

 Traced backwards, the olfactory tract is connected Avith the cerebrum 

 by three roots. The outer root passes, as a white streak, outwards and 

 backwards along the anterior margin of the perforated space, towards 



