112 THE CEREBRUM. 



extent. It is situated within the concavity of the corpus callo- 

 sum between the lateral ventricles, above and anterior to the 

 third ventricle. Below and posteriorly it is bounded by the fornix. 

 It is not a part of the embryonic brain cavity, but a mere lymph 

 space. Therefore it does not communicate with any other 

 ventricle, each of the others being a part of the cavity of the 

 neural tube from which both brain and cord are developed. 

 Instead of ependyma, which lines other ventricles, the lining of 

 the fifth is endothelium. A lymph-like fluid fills it. 



THE LATERAL VENTRICLE. 



( Ventriculus La teralis . ) 



The hemispheres contain the largest of the six ventricles (Figs. 

 30, 35, 41, 42 and 73). Situated one on either side of the median 

 line, the ventricles of the hemispheres are very naturally called 

 the lateral ventricles. Each represents a branch of the cavity 

 of the embryonic neural tube (Figs. 17 and 41). In consequence, 

 the lateral ventricles communicate with all others except the 

 fifth. By the interventricular foramen (of Monro), each directly 

 communicates with the third ventricle ; and through that, indirectly, 

 with the fourth and sixth. The foramen interventriculare is 

 situated between the front of the thalamus and the calumna of 

 the fornix (Fig. 29). It extends between the anterior extremity 

 of the third ventricle (the aula) and the junction of the anterior 

 horn with the central part of the lateral ventricle. The lateral 

 ventricles are lined with ependyma, which is a transparent mem- 

 brane composed of two layers when complete, viz., neuroglia 

 and a covering of columnar' ciliated epithelial cells. Over the 

 thalamus (the part seen in the lateral ventricle) and the chorioid 

 plexus, the neurogliar layer is absent. 



The lateral ventricle may be studied best in four parts: the 

 central part (or body); the anterior horn; the inferior horn; and 

 the posterior horn. 



The central part of the lateral ventricle (Figs. 36, 39 and 40) 

 is the ventricle of the parietal lobe of the cerebrum. The following 

 are its boundaries : 



Roof Corpus callosum. 



