636 THE NEKVOUS SYSTEM. 



centre of which it takes a curved course forwards and medially to a point about an 

 inch behind the extremity of the temporal pole. 



In the angle between the diverging posterior and descending horns the cavity 

 of the ventricle presents an expansion of a somewhat triangular shape. To this 

 the name of trigonum collaterale is sometimes given. 



The roof of the inferior horn is formed for the most part by the tapetum of 

 the corpus callosum. At the extremity of the horn the roof presents a bulging 

 into the cavity. This is produced by a collection of gray matter termed the 

 amygdaloid nucleus. The stria terminalis and the attenuated tail of the caudate 

 nucleus are both prolonged into the inferior horn and are carried forwards, in 

 its roof, to the amygdaloid nucleus. 



On the, floor of the inferior horn the following structures are seen : (1) 

 hippocampus; (2) the chorioid plexus; (3) the fimbria; and (4) the eminentia 

 collaterals. 



The hippocampus is for the most part covered by the chorioid plexus of the 

 lateral ventricle. If this is detached a fissure appears between the fimbria and the 

 roof of the ventricular horn. This is the chorioid fissure. It appears at a very 



Splenium of corpus callosum 

 Bulb of the posterior cornu Bulb of the posterior cornu 



', Fibres of corpus callosum (tape- 

 tum) 



'. Optic radiations 

 Inferior occipito-frontal fasciculus. 



FIG. 565. FRONTAL SECTION THROUGH THE POSTERIOR HORNS OF THE LATERAL VENTRICLES, 



VIEWED FROM THE FRONT. 



early date in the development of the cerebral hemisphere, and takes an arcuate 

 course round the posterior end of the thalamus. In the region of the pars 

 centralis of the lateral ventricle it extends as far forwards as the foramen inter- 

 ventriculare and is formed by the involution of an epithelial part of the wall of 

 the ventricle over the chorioid plexus (p. 622). In the region of the inferior horn, 

 when the chorioid plexus, with the involuted epithelial layer which covers it, is 

 withdrawn, the chorioid fissure is converted into an artificial gap which leads 

 directly into this part of the ventricular cavity. 



The chorioid plexus is a convoluted system of blood-vessels in connexion with a 

 fold of pia mater, which is prolonged into the inferior horn of the lateral 

 ventricle. It lies on the surface of the hippocampus and is continuous, behind 

 the posterior part of the thalamus, with the chorioid plexus in the pars centralis 

 of the lateral ventricle. But it must not be supposed that the chorioid plexus lies 

 free in the ventricular cavity. It is clothed in the most intimate manner by an i 

 epithelial layer, which represents the medial wall of the inferior horn involuted 

 into the cavity over the chorioid plexus. The ventricle, therefore, opens on the 

 surface only through the chorioid fissure when this thin epithelial layer is torn 

 away by the withdrawal of the chorioid plexus. From the above, it will be under- 



