744 



DISSECTION OF THE BRAIN. 



Dissection. 



Grey- 

 lamina. 



Corpus 

 callosum 



ends below 

 in two 

 bands, 



and extends 

 into hemi- 

 sphere. 



Vallecula 

 Sylvii. 



Anterior 



perforated 



space. 



Olfactory 

 lobe 



lies in 



olfactory 



sulcus. 



Olfactory 

 bulb. 



Olfactory 

 tract : 



roots, outer 



and inner. 



but in the foetus it is hollow, and the posterior lobe opens into tin; 

 third ventricle through the infundibulum. 



Dissection. To see the lamina cinerea and the anterior end of 

 the corpus callosum, the hemispheres are to be gently separated from 

 each other in front. 



The LAMINA CINEREA (fig. 268, m} is a thin concave layer of grey 

 substance, which gradually tapers upwards from the tuber cinereum 

 to the termination of the corpus callosum. This stratum closes the 

 anterior part of the third ventricle between the two central hemi- 

 spheres, and is continuous laterally with the anterior perforated 

 space. In consequence of its great thinness, this structure is often 

 broken through in removing the brain. 



The CORPUS CALLOSUM (ri), bent downwards in front, is continued 

 horizontally backwards in the longitudinal fissure to the lamina 

 cinerea, and ends by two white narrow bands the peduncles of the 

 corpus callosum (or sub-callosal convolutions), which are continued 

 along the edge of the lamina cinerea on each side to the anterior 

 perforated spot. The anterior bend of the corpus callosum is known 

 as the genu (fig. 274, p. 757), and the recurved portion is known as 

 the rostrum; but this, with the other parts of the corpus callosum, 

 will be seen to more advantage later. 



VALLECULA SYLVII AND ANTERIOR PERFORATED SPACE. The 

 vallecula Sylvii is a depression between the optic commissure and the 

 fore part of the temporal lobe of the hemisphere, which lodges the 

 upper end of the internal carotid artery. Externally it leads into 

 the Sylvian fissure, and in front it is bounded by the diverging roots 

 of the olfactory tract. The floor of the fossa is formed by a layer of 

 grey matter which is perforated by the central branches of the 

 anterior and middle cerebral arteries, thus giving rise to its name as 

 the anterior perforated space. 



The OLFACTORY LOBE consists of an elongated nerve-like part 

 which is named the olfactory tract, and a terminal expansion in front 

 the olfactory bulb. It lies in a groove (olfactory sulcus ; fig. 272, 

 p. 750, ol) on the surface of the inner orbital convolution of the 

 frontal lobe of the hemisphere. 



The olfactory bulb (fig. 268, o) is an oval mass, of a greyish colour, 

 and nearly half an inch in length, which rests on the cribriform 

 plate of the ethmoid bone. From its under surface the olfactory 

 nerves arise. 



The olfactory tract is a prismatic band, about an inch long, the 

 upper edge of which is received into the olfactory sulcus. It is 

 attached by its base, where it is somewhat expanded, to the frontal 

 lobe close in front of the anterior perforated space ; and from this 

 part two diverging white streaks, the inner and outer olfactory roots, 

 proceed to neighbouring convolutions. The external root passes 

 along the outer margin of the anterior perforated space, and across 

 the beginning of the Sylvian fissure, to the anterior extremity of the 

 temporal lobe. The internal root, not always visible, bends inwards, 

 and joins the lower end of the subcallosal convolution. By raising 



