THE LATERAL VENTRICLES 27 



is triangular in outline and its apex lies at the interventricular 

 foramina. It extends from the one side of the median plane 

 to the other and partially overlaps both thalami. In the 

 median plane the tela chorioidea is stretched across the gap 

 between the two thalami, and, in this situation, it forms the 

 roof of the third ventricle. Between its two layers the internal 

 cerebral vein (of Galen) passes backwards. It is formed at the 

 interventricular foramen by the union of the vein of the 

 corpus striatum (p. 29) with a vein from the chorioid plexus. 

 At the posterior end of the tela chorioidea the two internal 

 cerebral veins unite to form the great cerebral vein (of Galen] 

 (Fig. 15), which emerges below the splenium of the corpus 

 callosum and above the dorsal aspect of the mid-brain and 

 terminates in the straight sinus. 



Tumours of the cerebellum or of the corpora quadrigemina 

 may obstruct the great cerebral vein near its termination (Fig. 55) 

 and so produce engorgement of the veins of the chorioid plexus. 

 As a result of this venous stasis, an increased amount of serum 

 is transuded into the cerebral ventricles, giving rise to the 

 condition of acquired hydrocephalus. 



Prolongations of the lateral ventricle extend backwards into the 

 occipital lobe and downwards into the temporal lobe and form, 

 respectively, \he posterior and the inferior (descending) horns. 



The lateral ventricle may be tapped by passing in a special 

 trochar and cannula at a point two fingers'-breadth in front 

 of the mid-point of the line joining the glabella (p. 7) to the 

 external occipital protuberance and about the same distance 

 from the median plane (Fig. 16). The instrument is passed 

 downwards and backwards for from i^ to 2 inches before it 

 enters the ventricle (Kocher). The course taken by the 

 instrument is planned so as to avoid the motor cortex and 

 the middle frontal gyrus. 



The inferior horn may also be reached from the surface 

 without damage to the important cortical areas. The instru- 

 ment is inserted at a point two fingers'-breadth behind the 

 external acoustic meatus and the same distance above a line 



