28 



THE NERVOUS SYSTEM 



drawn backwards from the lower border of the orbit through the 

 centre of the external acoustic meatus (Fig. 16). It is directed 

 medially and slightly upwards and forwards and passes below 

 the higher auditory centres in the superior temporal gyrus. 

 In the normal subject the inferior horn lies at a depth of 



FIG. 16. Lateral Aspect of Skull, showing the relations of important 

 structures to the surface. 



12. Line drawn from floor of orbit through 

 centre of external acoustic meatus. 

 o 1 . Site for puncture of lateral ventricle. 



o 2 Site for puncture of inferior horn of 

 lateral ventricle. 



2 inches from the surface, but when the ventricles are dis- 

 tended, the brain substance may be greatly thinned out. 



The Third Ventricle, is situated in the median plane and 

 its lateral walls are formed by the two thalami. Anteriorly, 

 it is closed by the lamina rostralis, which extends from the 

 rostrum of the corpus callosum to the optic chiasma (Fig. 6). 

 This sheet of grey matter represents the anterior, or cephalic, 

 extremity of the primitive neural tube (p. 2). The floor of 

 the ventricle is formed by the structures which occupy the 



