186 THE CEKEBRAL HEMISPHERES. 



the two veins of Galen pass forwards from this point in the middle of the velum 

 interpositum, diverging somewhat behind, but again converging and ultimately 

 becoming united into a common trunk (vena magna Galeni, fig. 130, a) which opens 

 into the straight, sinus. In this course they receive several tributaries from the 

 optic thalami and other parts. 



Tela choroiclea inferior. This name has been given to the layer of pia mater 

 which, prolonged from the medulla oblongata, overlies the inferior half of the fourth 

 ventricle and is reflected at the margin of the velum medullare inferius on to the under 

 surface of the cerebellum. Like the velum interpositum, this also has two sets of 

 choroid plexuses, mesial and lateral, which are continuous with one another in front. 

 The mesial plexus extends forwards along either side of the middle line from the 

 foramen of Majendie (p. 188) to where" the tela is reflected along the edge of the 

 inferior medullary velum ; here the mesial plexuses are continued into the lateral 

 plexuses on either side, and these extend to the apertures (in the pia mater) of the 

 lateral recesses of the ventricle (p. 48). 



On the spinal 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 an external fibrous layer, which is arranged in longitudinal 

 glistening 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 cranium, the pia mater becomes continuous with their 

 connective tissue sheaths. 



The pia mater of the cord is thickened by a conspicuous fibrous band, running 

 down in front over the anterior median fissure. This was named by Haller, linea 

 splendens. 



Structure. The pia mater of the cord consists of two layers, the outer one 

 being composed of interlaced bundles of connective tissue, which are for the most 

 part parallel and longitudinal, and the inner or intima of peculiar stiff bundles 

 bending suddenly and enclosing somewhat angular interspaces. Both surfaces of 

 this inner layer are covered with endothelial cells, and there is a network of fine 

 elastic fibres near the surfaces. On the cord pigmented cells are sometimes scattered 

 among the elastic fibres. The outer and inner layers are separated here and there 

 by cleft-like lymphatic spaces communicating on the one hand with the subarachnoid 

 space and on the other with the perivascular canals immediately to be mentioned. 

 In the pia mater of the brain only the inner of the two layers of the pia mater of 

 the cord is represented. 



The choroid plexuses are beset with a large number of highly vascular 

 villous prolongations of the pia mater (choroidal villi), the larger of which are from 

 1 mm. to 2 mm. long, but are subdivided into smaller secondary or even tertiary 

 villi. Each larger villus has an afferent artery and efferent vein which open into a 

 capillary network lying close to the surface. The free surface of the villi and of- the 

 depressions between them is covered everywhere by a simple flattened or cubical 

 epithelium, which is ciliated in lower vertebrates, but in mammals is said to possess 

 cilia only in embryonic life. Each cell very commonly contains a yellowish fat 

 globule. 



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

 before they enter the nervous substance. In the pia mater of the cord they lie 

 between its two layers, but in that of the brain on the surface of the membrane, 

 either projecting freely or covered by subarachnoid trabeculae. Further each vessel 

 is enclosed by a sheath composed of a more dense arrangement of the fibres of the 



