NERVOUS SYSTEM 197 



In consequence of this process of differentiation, the originally 

 single perimedullary space is now represented by three lymph 

 spaces : a peridural, a suldural, and a subarachnoid space (c). The 

 last mentioned encloses a network of trabeculye. 1 



The brain-membranes are formed in essentially the same way 

 as those of the spinal cord. The dura mater, however, possibly owing 

 to the rapid growth of the brain, becomes pressed against the 

 periosteum (so-called " endocranium" corresponding to the 

 endorachis of the spinal column), with which it may become fused 

 (e.g. Mammals) and thus was formerly known as the outer layer of 

 the dura. 2 



In Fishes and tailed Amphibians ; the whole subdural space is 

 filled by a loose tissue consisting of meshes enclosing lymphoid 

 and fatty tissue, while in Anurans this is only the case in the 

 anterior part of the skull. Further back, to the end of the spinal 

 canal, there is a definite continuous space filled with lymph, and 

 this is especially well developed on the dorsal side of the brain 

 and spinal cord. 



Functionally, the dura serves as a kind of internal periosteum, 

 while the vascular pia is important in connection with the 

 nutrition of the central nervous system. In places where the 

 walls of the brain are epithelial, and do not become converted into 

 nervous tissue, the pia also takes part secondarily in lining 

 the ventricles, into which it may extend, pushing the epithelial 

 wall (ependyme) before it, and thus giving rise to telce or plexus 

 choruidci, which are very important throughout the Vertebrate 

 series. 3 



The arachnoid, which, as mentioned above, becomes differenti- 

 ated between the dura and pia, is not really a membrane : it 

 consists of an extensive system of meshes of lymphoid (adenoid) 

 tissue, the cavities in which are lined by an epithelium and con- 

 tain a serous or lymphoid fluid. The meshes bridge over all 



1 The cause of this gradual increase in complication is to be traced in the first 

 instance to the increasing vascularisation of the spinal cord in passing from the 

 lower to the higher Vertebrates : this results in an increase in the quantity of 

 lymph and necessitates an arrangement of larger channels for carrying it off. 



2 The morphological difference between these two membranes is seen most 

 plainly at the optic foramen, at which the periosteum of the cranium is directly 

 continuous with that of the orbit, while the dura proper forms the sheath of the 

 optic nerve. The mode of origin of the so-called sinus durcK matris is not known 

 with certainty, but they appear to have nothing to do with the dura proper and to 

 correspond to sinuous enlargements of the periosteal " endocranium," as is indicated 

 by similar spaces in the endorachis of Anamnia, Reptiles, and mammalian embryos, 

 which are especially well seen around parts of the brain in Urodeles. In Anurans 

 they even extend beyond the cranial cavity along the whole length of the spinal 

 canal. (Cf . under Auditory Organ perilymphatic and endolymphatic ducts. ) 



:: The brain-membranes in Sauropsida require further investigation. In 

 Mammals the dura gives rise to folds extending between parts of the brain, known 

 as the frt/:c and the t< nforiiim. The former, slight indications of which are also 

 -(en in Birds, extends between the two hemispheres, the latter between the hind- 

 brain and the occipital lobes of the hemispheres : both may become ossijied 

 I- ..'/. in C'arnivora). 



