THE ARACHNOID ■ 917 



there are, as a rule, two emissary veins: — one, the mastoid emissary vein, which passes through 

 the mastoid foramen to the occipital or posterior auricular vein; and the other, the post-condy- 

 loid vein, which traverses the condyloid (posterior condyloid) foramen and joins the suboccipital 

 plexus. The cavernous sinus is in communication anteriorly with the superior ophthalmic 

 vein, and through the latter with the angular vein; it is connected with the pterygoid plexus 

 by emissary veins which pass either through the foramen ovale or the foramen Vesalii, and 

 with the pharyngeal plexus by small venous channels which accompany the internal carotid 

 artery through the carotid canal. 



The venous lacunse or spaces are small clefts lined by endothelium which communicate 

 with the meningeal veins and with the blood sinuses. They also have communications with 

 the emissary veins and the diploic veins. They lie between the outer and inner layers of the 

 dura mater, the majority of them at the sides of the superior sagittal sinus, but others are found 

 in the tentorium associated with the transverse sinuses and the straight sinus. 



Blood-vessels. — The blood supply of the cranial dura mater is derived from the meningeal 

 arteries, which ramify in its outer layer. The more important of these arteries have already 

 been described in the account of the vascular sj-stem, and it is only necessary here to recall 

 the fact that the greater part of the dura mater above the tentorium cerebelli is supplied by 

 branches of the middle meningeal arteries. These are reinforced — (1) at the vertex by branches 

 of the occipital arteries which enter through the parietal foramina; (2) in the middle fossa by 

 the small meningeal arteries and by meningeal branches of the internal carotid, lacrimal, 

 and ascending pharjmgeal arteries; and (3) in the anterior fossa by meningeal branches of the 

 anterior and posterior ethmoidal arteries. 



The dura mater in the posterior fossa of the skull, below the tentorium cerebelU, also re- 

 ceives branches from the middle meningeal arteries, but its blood supply is derived mainly — (1) 

 from the meningeal branches of the vertebral arteries which enter the fossa through the foramen 

 magnum, (2) from meningeal branches of the occipital arteries which enter through the mastoid 

 and h^'poglossal foramina, and (3) from meningeal branches of the occipital and ascending 

 pharyngeal arteries which enter through the jugular and hypoglossal (anterior condyloid) 

 foramina. 



The meningeal veins accompany the arteries as vence comitantes, usually one vein with each 

 artery. The middle meningeal artery usually has two venae comitantes. The meningeal veins 

 communicate with the venous sinuses and with the diploic veins, and, unlike ordinary veins, 

 they do not increase much in calibre as they approach their terminations. 



The 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 

 filaments, which supply the cranial dura mater are sensory fibres derived from the trigeminus 

 and vagus nerves, and possibly from the first cervical nerves. The branches from the trigeminus 

 are derived from the three divisions of that nerve on each side, and it has been stated that 

 branches are given from the nasal branch of the ophthalmic division to the dura mater in the 

 anterior fossa. 



The meningeal branch of the ophthalmic division of the trigeminus supplies the tentorium; 

 that from the maxillary division accompanies the branches of the middle meningeal artery. 

 The meningeal branch of the mandibular division (nervus spinosus) passes into the skull through 

 the foramen 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 fossa of the cranium. This recurrent or meningeal branch of the hypo- 

 glossal nerve reallj^ consists of fibres derived from the superior cervical ganglion of the sympa- 

 thetic, and contains sensory fibres from the first and second cervical nerves. The meningeal 

 branch of the vagus springs from the ganglion of the root of that nerve, and is distributed in the 

 posterior cranial fossa. The sympathetic filaments are distributed to the smooth muscle of the 

 walls of the blood-vessels. 



The cranial subdural cavity is not of uniform thiclvness tliroughout, being 

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

 usually but little more than 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 continuous 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-hke delicacy, which in reality pervades the space it occupies. 



Its outer surface, or that closely related to the dura mater and bounding the subdural cavity 

 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 endothelial cells covering the blood-vessels, 



