THE NERVOUS SYSTEM 1473 



Veins of the Diplo'e. — These vessels axe situated in the cancellated tissue 

 between the outer and inner plates of the cranial bones, and are exposed after 

 removal of the outer plate. They are destitute of valves, and are arranged 

 in the form of a network, from which the blood is returned by four diploic 

 veins on either side, namely, frontal, anterior temporal, posterior temporal, 

 and occipital. These terminate partly in extracranial veins, and partly in 

 the intracranial venous sinuses and meningeal veins. 



The frontal diploic vein passes downwards, and escapes through an open- 

 ing in the outer plate of the frontal bone at the supra-orbital notch, where 

 it joins the communicating vein which passes between the supra -orbital and 

 ophthalmic veins. It receives radicles from the frontal air-sinus of the 

 same side. 



The anterior temporal diploic vein is confined to the back part of the 

 frontal and anterior part of the parietal regions, and descends to terminate 

 in two ways. It partly joins one of the extracranial deep temporal veins, 

 by passing through an opening in the outer plate of the great wing of the 

 sphenoid, and in part it ends in a meningeal vein, or in the spheno-parietal, 

 or, it may be, the cavernous, sinus. 



The posterior temporal diploic vein is confined to the parietal region, and 

 descends to the postero-inferior angle of the parietal bone, where it pierces 

 the inner plate of that bone, and terminates in the lateral sinus. 



The occipital diploic vein occupies the occipital region, and pierces the 

 inner table of the occipital bone, to terminate in the lateral sinus. 



Nerves of the Dura Mater. — The dura mater receives nerves from (i) the 

 sympathetic plexuses which accompany the arteries ; (2) the three divisions 

 of the fifth cranial nerve ; (3) the ganglion of the root of the pneumogastric 

 nerve ; and (4) the hypoglossal nerve. 



Processes cf the Dura Mater. — The dura mater, as stated, is com- 

 posed of two layers — outer or periosteal, and inner or sustentacular. 

 The processes are formed by the inner or sustentacular layer, and are 

 four in number, namely, the falx cerebri, tentorium cerebelli, falx 

 cerebelli, and diaphragma sellse. 



The falx cerebri is an extensive falciform process, which occupies 

 the great longitudinal fissure of the cerebnmi, where it lies between 

 the two cerebral hemispheres. Anteriorly it is almost pointed, 

 and is attached to the crista galli of the ethmoid bone. Posteriorly 

 it is broad, and is attached to the upper surface of the tentorium 

 cerebelli along the median line, the straight sinus being situated 

 at the place of junction. The superior border is convex, and is 

 attached in the median line to the frontal, parietal, and occipital 

 bones, extending upon the latter bone only as low as the internal 

 occipital protuberance. The superior longitudinal sinus is situated 

 along this border. The inferior border is concave and free. It 

 overhangs the upper surface of the corpus callosum. from which 

 it is separated by a slight interv^al. The inferior longitudinal sinus 

 is situated within this border. The lateral surfaces face the mesial 

 surfaces of the cerebral hemispheres. 



The tentorium cerebelli is an extensive crescentic sheet which 

 covers the cerebellum. Superiorly it supports the posterior parts 

 of the cerebral hemispheres, and is elevated along the median line, 

 whence it slopes towards the attached borders. Anteriorly it 

 presents a free, sharp, concave border, which forms, with the 

 dorsum sellae of the sphenoid bone anteriorly, an oval opening, 

 called the incisura tentorii, within which the mesencephalon is 



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