THE NERVOUS SYSTEM 413 



This is seen almost everywhere, but the extent of the develop- 

 ment in thickness varies much. It is well shown by the thick- 

 ening of the floor of the telencephalic lobes, forming the 

 corpora striata, or by that of the roof and sides of the same 

 parts in the higher vertebrates, forming the cerebral hemi- 

 spheres. 



2. The retention of the embryonal thinness over a definite 

 area, forming a place where the lumen is separated from the 

 exterior by merely a thin, often a transparent, membrane. Such 

 places are extremely puzzling, and misled anatomists until 

 within a generation. The physiological purpose of such a thin 

 place is to allow the blood to communicate with the lymph of 

 the ventricles and to nourish the inner surfaces without violat- 

 ing the integrity of the original neural tube. A plexus of 

 blood-vessels is thus the constant accompaniment of such a 

 thin place, and the relations usually become still more compli- 

 cated by the sinking into the cavity of the entire structure, 

 although each loop of capillaries is covered and veiled by the 

 membranous wall, and thus the integrity of the tube is never 

 violated.* In extreme cases almost the entire thin area, cov- 

 ering a network of capillary loops and following its intricacies, 

 may come to lie within the cavity of a ventricle and form a so- 

 called chorioid plexus. 



The most important of these organs are (i) those of the 

 lateral ventricles, formed by the imagination of a thin area in 

 the inner wall of each, (2) a similar one in the third ventricle, 

 invaginated from its roof, and (3). one formed from the roof 

 of the fourth ventricle immediately behind the cerebellum. 



* As the only exception to this rule there have been described in Man 

 and in certain other mammals one or more small perforations in the roof 

 of ihe fourth ventricle, the foramina of Majendie, which form a direct 

 communication between the lumen of the neural tube and the sub- 

 arachnoid space. The existence of this communication, which violates 

 the morphological principle of the complete integrity of the walls of the 

 neural tube, has given rise to much discussion, but it seems now probable 

 that, while these foramina certainly do occur occasionally, it is an individual 

 peculiarity, like the epitrochlear foramen of the humerus, and of no 

 especial significance. 



