MEMBRANES OF BRAIN AND SPINAL CORD 151 



In an early stage of development the lurnen of the medullary 

 cord is primitively continuous posteriorly with that of the primary 

 intestine (neurenteric canal). This connection, however, soon dis- 

 appears, and the cord then consists of a cylindrical or more or less 

 flattened hollow cord with thick walls, the cavity of which is lined 

 by ciliated epithelium and expands in front to form the ventricles of 

 the brain. This cavity becomes greatly reduced later, and in the 

 spinal cord is spoken of as the central canal. 



Membranes of the Brain and Spinal Cord. 



The enveloping membranes of the brain and spinal cord arise 

 by the differentiation of a connective-tissue layer lying between the 

 central organs of the nervous system and the surrounding skeletal 

 parts. In Fishes, only two membranes are distinguishable : one, 



FIG. 122. BRAIN MEMBRANES OF MAN. (After Schwalbe.) 



DM, dura mater ; SR, sub-dural (arachnoid) space ; A, sub-arachnoid space ; PM, 

 pia mater ; OR, gray cortical substance of the brain. 



the dura mater, lining the inner surface of the cerebro-spinal 

 canal, and the other, or pia mater, investing the brain and spinal 

 cord. The latter represents also the arachnoid of higher Verte- 

 brates, which is not here differentiated as a separate membrane. 

 The dura mater conveys vessels to the walls of the cerebro-spinal 

 canal that is, to the perichondrium or periosteum, while the pia 

 mater, which is much richer in blood-vessels, has to do with the 

 nutrition of the nervous axis. The dura mater consists of two 

 lamellae, which, however, only remain distinct along the whole 

 central nervous system in the lower Vertebrata. In higher Verte- 

 brates, its double nature persists only in the region of the vertebral 

 column, the two layers becoming fused in the cranial portion. As 

 in most Fishes the brain by no means fills the cranial cavity, a large 

 lymph-space lies between the dura and pia mater; this cor- 

 responds to the so-called sub-dural space of terrestrial Vertebrates. 



