572 MEMBRANES OF BRAIN AND CORD. 



contain a double layer. From its internal Kiu-face numerous small 

 vessels enter the substance of the brain, and hence this inner surface 

 is very flocculent, and is named tomentum cerebri. On the cerebellum 

 a similar arrangement exists, but the membrane is finer, and the 

 double fold only distinct in the larger sulci. The pia mater is also 

 prolonged through the transverse fissure into the lateral ventricles, and 

 there forms the velum interpositum and choroid plexus. It is also 

 prolonged into the fourth ventricle, where it forms the choroid plexus 

 of the fourth ventricle. 



On the sp'mal cord the pia mater has a very different character from 

 that which it presents on the encephalon, so that it has even been 

 described by some as a different membrane under the name neurilemma 

 of the cord. It is thicker, firmer, less vascular, and more adherent to 

 the subjacent nervous matter : its greater strength is owing to its 

 containing fibrous tissue, which is arranged in longitudinal shining 

 bundles. A fold of this membrane dips down into the anterior fissure 

 of the cord, and serves to conduct blood-vessels into that part. A 

 thinner process passes into the greater part of the posterior fissure. 

 At the roots of the nerves, both in the spine and in the cranimn, the 

 pia mater becomes continuous with the neurilemma. 



The pia mater of the cord presents a conspicuous fibrous band, run- 

 ning down in front over the anterior median fissure. This was named 

 by Haller, linea splendcns. 



Structtire. — The pia mater consists of interlaced bundles of connec- 

 tive tissue, having a more regular arrangement in the outer and inner 

 layers, while in the middle is a network of fine elastic fibres. On the 

 cord the outer fibres are for the most part parallel and longitudinal, 

 and the inner network consists of peculiar stiff fibres bending sud- 

 denly and enclosing angular interspaces. Beneath them is a fine mem- 

 brane continuous with the neuroglia of the cord. On the cord 

 pigmented cells are sometimes scattered among the elastic fibres. The 

 pia mater contains great numbers of blood-vessels, which subdivide in 

 it before they enter the nervous substance. Each vessel lies in a 

 canal, the walls of which are composed of a more dense arrangement 

 of the fibres of the membrane (perivascular canal). The diameter of 

 the canal may be two or three times that of the contained vessel. A 

 similar sheath, derived from the pia mater, accompanies the vessel into 

 the substance of the brain. At its commencement it is loose and funnel- 

 shaped and can be injected from the subarachnoid cavity. On the 

 cerebrum the inner layer of the pia mater is adherent to the cortical 

 substance of the convolutions, but on the cerebellum a space exists 

 between the two, traversed by fibres which pass from the cerebellum to 

 the pia mater. This space is continuous with the intervals between the 

 perivascular sheaths and the brain substance. 



According to Fohmann and Arnold, the pia mater contains numerous 

 lymphatic vessels. Purkinje describes a retiform arrangement of 

 nervous fibrils, derived, according to KoUiker and others, from the sym- 

 pathetic, the third, sixth, facial, pneumogastric, and accessory nerves. 

 The spinal pia mater is supplied by nerves from the sympathetic. 



THE AKACHNOID MEMBKANE. 



The arachnoid is a delicate membrane which invests the brain and 

 spinal cord, outside the pia mater, and much less closely than that mem- 



