THE NERVOUS SYSTEM I47» 



The Pacchionian bodies are enlargements of the vilh of the 

 aiachnoid membrane (see p. 1481). 



Stractare. — The dura mater consists of fibrous and elastic tissues, arranged 



as parallel bundles. 



The intracranial dura mater differs from the dura mater of the 

 spinal cord in the following respects : (i) it consists of two layers — 

 outer or periosteal, and inner or sustentacular — ^whereas the spinal 

 dura mater has only one layer, representing the sustentacular 

 layer; (2) it furnishes certain processes or septa, which project into 

 the cranial cavity, and separate parts of the encephalon from each 

 other, whereas the spinal dura mater sends no septa into the spinal 

 cord ; and (3) it contains venous sinuses, which are absent in the 

 spinal dura mater. 



Subdural Space. — ^This is the interval between the dura mater and 

 the arachnoid membrane. There is really no space, but simply 

 sufficient interval to contain a minute quantity of serous fluid for 

 lubricating purposes. The dura mater and the arachnoid are there- 

 fore practically in contact with each other. 



Blood-supply. — The cranial dura mater is supplied by the meningeal 

 arteries, which are also distributed to the inner table and diploe of the cranial 

 bones. These vessels cire very numerous, and the chief are as follows on 

 either side, from before backwards : (i) anterior meningeal, two in number, 

 from the anterior ajid posterior ethmoidal arteries ; (2) meningeal, from the 

 cavernous part of the internal carotid artery ; {3) small meningeal, from the 

 middle meningeal, or from the first part of the internal maxillary artery ; 

 (4) middle meningeal, from the first pjui: of the internal maxillary ; (5) menin- 

 geal branches of the ascending pharjmgeal artery ; (6) posterior meningeal 

 branch of the occipital artery ; and (7) posterior meningeal, from the vertebral 

 artery. 



The anterior meningeal branch of the anterior ethmoidal artery arises from 

 that vessel as it accompanies the nasal nerve on the cribriform plate of the 

 ethmoid bone, and it takes part in the supply of the dura mater of the anterior 

 fossa. 



The anterior meningeal branch of the posterior ethmoidal artery arises from 

 that vessel after it has entered the cranial cavity through a minute foramen 

 between the cribriform plate of the ethmoid and the sphenoid. It has a 

 limited distribution to the dura mater in this region. 



The meningeal branch of the internal carotid artery arises from the cavernous 

 part of that vessel, and enters the middle fossa to supply the dura mater. 



The small meningeal artery is usually a branch of the middle meningeal, 

 but it may arise from the first part of the internal maxillary artery. It 

 enters the cranial cavity through the foramen ovale, and supplies the adjacent 

 dura mater and the Gasserian ganglion. 



The middle meningeal artery, as stated, is a branch of the first part of the 

 internal maxillary artery. It is of large size, and enters the cranicd cavity 

 through the foramen spinosum in the sphenoid bone, after which it divides 

 into two branches, anterior and posterior. The anUrior branch passes for- 

 wards, outwards, and upwards in a groove on the upper surface of the great 

 wing of the sphenoid bone to the inner aspect of the antero-inferior angle of 

 the parietal bone, where there is a groove, or sometimes a short canal. The 

 position of the middle meningeal sirtery at this level is ascertained by taking 

 a point on the exterior of the skull i^ inches behind the external angular 

 process of the frontal bone, and i^ inches above the zygomatic arch. From 

 this point the artery ascends in a branching meningeal groove near the 

 anterior border of the parietal bone, as high as the superior longitudinal sinus. 



