THE ARACHNOID 891 



The dura TM.-itcr in the posterior fossa of (ho skull, below Ihc tentorium coroliolli. also receives 

 branches Irom the middle moningeal arteries, but its blood supply is derived mainly (1) from 

 tin' mcninnoal brandies of the vertebral arteries which enter the lossa through the foramen mag- 

 mini, J no in memnireal brandies ol the occipital arteries whic-li enter through the mastoid 

 and COndyloid loramma. and (.'ii from meningeal branches of the posterior auricular and as- 

 cending pharvnijcal arteries which enter through the jugular and hypoglossal (anterior eondy- 

 luid foramina. 



The meningeal veins accompany the art' vena romiliK, usually one vein with . 



artery. Ihc middle menmgeal artery usually has two veme eoinitos. The meninircal veins 

 communicate with the venou.-, sinuses and with the diploic veins, and, unlike ordinary veins, 

 t|,,.\ !" not increase much in calibre as the}- approach their terminations. 



I he nerves of the dura mater are partly derived from the sympathetic filaments which 

 accompany the arteries and partly from the cranial nerves. The nerves, other than sympathetic 

 ti laments, which supply the cranial dura mater are derived from the fifth and tenth cranial nerves, 

 and possibly from the ' tir-i cervical nerve. The branches from the trigeminus are derived from 

 the lir-t,.-econd and third divisions of that nerve on each side, and it lias been stated that brail' 

 are given from the nasal branch of the first division to the dura mater in the anterior fossa. 



The mcningeal branch of the first, division of the trigeminus supplies the tentorium; that 

 from (lie second di\ision accompanies the branches of the middle meningeal artery. The 

 meningeal branch of the third division (nerviis spinosus) passes into the skull through the fora- 

 men spinosum and is distributed to the dura mater over the great wing of the sphenoid and 

 to the mastoid cells. The recurrent branch of the hypoglossal nerve passes to the dura mater 

 of the posterior t'o^a of the cranium. It is probable that this recurrent or meningeal branch of 

 the twelfth nerve really consists of fibres derived either from the superior cervical ganglion of 

 the sympathetic or from the first and second cervical nerves. The meningeal branch of the 

 tenth nerve (vagus) springs from the ganglion of the root of that nerve, and is distributed 

 in the posterior cranial fossa. 



The cranial subdural cavity is not of uniform thickness throughout, being thinner 

 along the basal aspect of the encephalon. The lymph contained in it is usually but 

 little more ihan is sufficient to keep moist its bounding surfaces. It is continuous 

 with the lymph capillaries of the nerves and those of all the tissues it bathes, and it is 

 in communication with the similar cavity of the spinal canal. Its lymph is in free 

 contact with the blood-vessels passing through it and with those in the tissues it 

 bathes, and it is replenished by filtration through their walls. Though extensive, 

 the subdural space is thin at best, for the dura mater is quite closely applied to the 

 second of the three meninges. 



THE ARACHNOID 



The arachnoid or 'serous' membrane is the middle of the three meninges of the 

 central nervous system. As in the case of the other two, an attempt is made to 

 give this membrane a name descriptive of its texture. It is a gauzy reticulum of 

 almost web-like delicacy, which in reality pervades the space it occupies. Its outer 

 surface, or that closely related to the dura mater and bounding the subdural cav- 

 ity, alone shows a sufficiently organized structure to merit the name of membrane. 

 This surface is covered by a layer of endothelium which is identical with that lining 

 the inner surface of the dura mater and is continuous with it by way of the endothe- 

 lial cells covering the blood-vessels, the nerve-roots, the ligamenta denticulata of the 

 spinal cord, and the occasional delicate t.rabeculse passing between the dura mater 

 and t he arachnoid. Immediately under the endothelium, the connective-tissue fibres 

 of the arachnoid are woven into a very thin, more or less compact feltwork. This, 

 however, quickly grades into a loose, spongy reticulum which pervades the thick 

 aubaracbnoid cavity throughout, and the strands of which are directly continuous into 

 t be more compact t issue of the pia mater. Thus an inner surface can hardly be claimed. 

 This loose, sponge-like arachnoid tissue holds the cerebro-spinal fluid of the sub- 

 arachnoid cavity, the meshes of the sponge constituting a retieular web of intercom- 

 municating spaces lined by endothelioidal cells covering the strands of the web. The 

 cranial subaraehnoid cavity is larger, and the strands of the web are relatively more 

 abundant than in that of the spinal canal. In addition, the cavity is traversed by the 

 spinal and cranial nerves, by t lie blood-vessels passing to and from the pia, and, in the 

 spinal canal distinctively, it is traversed by the ligamenta denticulata and thefilum 

 terminale. Through these the arachnoid is further continuous with the pia mater. 



The cranial arachnoid is directly continuous into that of the spinal cord, and in 

 the two localities does not differ as much as does the dura mater. Within the 

 cranium, the arachnoid does not closely follow the surface of the encephalon. It is 

 folded in between the cerebellum and cerebral hemispheres, following the contour of 

 the tentorium cerebelli, but it does not dip into the fissures and sulci except the ante- 



