564 CEREBRO-SPINAL MEMBRANES. 



in all directions towards its circumference, thus corresponding in form with 

 the upper surface of the cerebellum. Its inner border is free and concave, and 

 leaves in front of it an oval opening, through which the isthmus encephali 

 descends. It is attached behind and at the sides by its convex border to 

 the horizontal part of the crucial ridges of the occipital bone, and there 

 encloses the lateral sinuses. Farther forward it is connected with the upper 

 edge of the petrous portion of the temporal bone the superior petrosal 

 fcinus running along this line of attachment. At the point of the pars 

 petrosa, the external and internal borders meet, and may be said to inter- 

 sect each other the former being then continued inwards to the posterior, 

 and the latter forwards to the anterior clinoid process. 



The falx cercbelli (falx minor) descends from the middle of the posterior 

 border of the tentorium with which it is connected, along the vertical ridge 

 named the internal occipital crest, towards the foramen magnum, bifurcating 

 there into two smaller folds. Its attachment to the bony ridge marks the 

 course of the posterior occipital siuus, or sinuses. 



Structure. The dura mater consists of -white fibrous and elastic tissue, arranged in 

 bands and laminae, crossing each other. It is traversed by numerous blood-vessels 

 which are chiefly destined for the bones. Minute nervous filaments, derived from 

 the fourth, fifth, and eighth cranial nerves, and from the sympathetic, are described 

 as entering the dura mater of the brain. Nervous filaments have likewise been 

 traced in the dura mater of the spinal column. (Luschka and Riidinger, quoted by 

 Hyrtl.) 



THE PIA MATER. 



The pia mater is a delicate, fibrous, and highly vascular membrane, 

 which immediately invests the brain and spinal cord. 



Upoii the hemispheres of the brain it is applied to the entire cortical 

 surface of the convolutions, and dips into all the sulci. From its internal 

 surface very numerous small vessels enter the grey matter and extend for 

 some distance perpendicularly into the substance of the brain. The inner 

 surface of the cerebral pia mater is on this account very flocculent, and is 

 named tomentum ceiebri. On the cerebellum a similar arrangement exists, 

 but the membrane is finer and the vessels from its inner surface are not so 

 long. The pia mater is also prolonged into the ventiicles, and there forms 

 the velum interpositum and choroid plexus. 



Structure. The pia mater consists of interlaced bundles of areolar tissue, con- 

 veying great numbers of blood-vessels ; and, indeed, its peculiar office, both on the 

 brain and spinal cord, seems to be that of providing a nidus or matrix for the 

 support of the blood-vessels, as these are subdivided before they enter the nervous 

 substance. According to Fohmann and Arnold, it contains numerous lymphatic 

 vessels. Purkinje describes a i etiform arrangement of nervous fibrils, derived, accord- 

 ing to Kolliker and others, from the sympathetic, the third, sixth, facial, pneumo- 

 gastric. and accessory nerves. 



On the spinal cord the pia mater has a very different structure from that 

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

 Home as a different membrane under the name neurilcmma of Hie 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 reduplication of this mem- 

 brane 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 



