290 INNER, VATION. [CHAP. X. 



extends into the anterior lobe (anterior or frontal horn), and into 

 the posterior lobe (posterior or occipital horn). The central part of 

 the horizontal portion is separated from the third ventricle by the 

 body of the fornix. In this portion of the ventricle are seen the 

 npper surfaces of the corpus striatum and optic thalamus, with the 

 tsenia semicircularis between them, covered by the lamina cornea. 

 The thalamus is partly concealed by the choroid plexns. The 

 descending or inferior horn (sphenoidal horn) communicates with 

 the body of the ventricle just behind the corpus striatum, and from 

 that point passes downwards and outwards, and then forwards and 

 inwards. It contains a remarkable convolution, the hippocampus 

 major, which projects into it, and is a continuation of that enclosing 

 the superior longitudinal commissure ; this is covered by an expan- 

 sion of fibrous matter continuous with the posterior pillars of the 

 fornix. The posterior horn contains a similar but smaller convo- 

 lution, called hippocampus minor. 



The inferior horn of the lateral ventricle contains a considerable 

 portion of the choroid plexus. This enters at its inferior extremity 

 between the hippocampus major and the crus cerebri, and passes 

 upwards into the horizontal portion of the ventricle. 



The fourth and the fifth ventricles have been already described. 



All the ventricles are lined by a very delicate membrane, similar 

 in structure to serous membrane. It is covered by a fine epithe- 

 lium consisting of polygonal scales, and provided with cilia, which 

 were first observed by Purkinje and Valentin. This epithelium 

 is found covering the surface not only of the wall of the ventricle, 

 but also of the pia mater within it, the choroid plexuses, and the 

 deep surface of the velum interpositum. It is by means of the 

 reflection of this membrane upon the intraventricular processes 

 of pia mater, that the ventricles are closed at those points where 

 nervous matter does not exist, such as the inferior cornua of the 

 lateral ventricles, and the inferior extremity of the fourth ventricle. 

 There is, therefore, no direct communication of these cavities at 

 these points with the subarachnoid space ; and, if fluid pass from 

 one to the other, it must be by filtration through the delicate 

 ventricular membrane. 



In a state of health, there is little or no fluid in the lateral or 

 other ventricles of the brain. Their inner surfaces are doubtless in 

 contact, but lubricated with a moisture, as all serous surfaces are. 

 When fluid is found in them, it results either from changes which 

 take place after death, or from some morbid process during life. 

 A state of anseinia, or an impoverished condition of the blood, in 



