468 THE TISSUES. 



of the finest fibres crossing each other in various directions. (Fig. 

 311.) Many of these fibres also form loops, and return into the 

 gray-red substance, again, as first shown by Valentin (p. 448). 



Kolliker has not been able to discover any connection between 

 these fibres and the cells of the gray matter of the convolutions ; 

 though the existence of such a connection is nowhere else so pro- 

 bable as here. Doubtless the nerve-fibres originate here, if any- 

 where in the central organs. Professor Domrich has, however, 

 traced the many -rayed cells of the cerebellum into nerve-fibres, as 

 he asserts ; and Kolliker has found divisions of the nerve-fibres in 

 the cord, but never in the encephalon. It is probable that the 

 fibres of the corpus callosum and the commissural fibres in general, 

 commenck in cells in one hemisphere and terminate in the other. 



Gray matter is also found in the cerebrum at other points ; viz., 

 in the anterior portions of the body of the corpus callosum, above 

 the septum lucidum, the fornix, and the corpus striatum; occasion- 

 ally on the surface of the corpus callosum between the raphe and 

 the striae, and which is continued into the fascia dentata of the pes 

 hippocampi, and in the hippocampus itself. 



THE MEMBRANES AND VESSELS OF THE NERVOUS CENTRES. 



The whole cerebro-spinal axis, just described, is inclosed in three 

 membranes; 1, the internal, or pia mater ; 2, the middle, or arach- 

 noid; &ndi 3, the external, the dura mater. 



1. The pia mater is the vascular membrane of the nervous cen- 

 tres. It is composed of collagenous tissue (p. 279, 6), and conducts 

 the vessels into the nervous substance. Hence it is in intimate 

 contact everywhere with the cord, and covers all the elevations and 

 depressions on the surface of the encephalon ; excepting alone the 

 floor of the fourth ventricle, above which it stretches across. On 

 the cerebrum it is more Vascular and more delicate than upon the 

 cord. It penetrates into the brain only at one point, viz., the trans- 

 verse fissure of the cerebrum where, under the name of the velum 

 interpositum it invests the vena magna Galeni and the pineal body ; 

 then forms the tela choroidea superior, the choroid plexus of the third 

 ventricle, and the vascular plexuses of the lateral ventricles, which are 

 continuous with the pia mater at the base of the brain. It con- 

 tains fusiform, bright-yellow, or brown pigment-cells, both in its 

 spinal and its encephalic portions. They are so abundant in the 

 cervical region as not unfrequently to give the membrane a brown 



