THE BRAIN AND ITS MEMBRANES. 703 



MEMBRANES OF THE BRAIN. 



The membranes of the brain are the dura mater, arachnoid membrane, and 

 pia mater. 



The Dura Mater. 



The Dura Mater (Fig. 410) is a thick and dense inelastic fibrous membrane which 

 lines the interior of the skull. Its outer surface is rough and fibrillated, and adheres 

 closely to the inner surface of the bones, forming their internal periosteum, this 

 adhesion being most marked opposite the sutures and at the base of the skull. Its 

 inner surface is smooth and lined by a layer of endothelial cells. It sends three 

 processes inward, into the cavity of the skull, for the support and protection of 

 the different parts of the brain, and is prolonged to the outer surface of the skull 

 through the various foramina which exist at the base, and thus becomes continuous 

 with the pericranium ; its fibrous layer forms sheaths for the nerves which pass 

 through these apertures. At the base of the skull it sends a fibrous prolongation 

 into the foramen caecum ; it sends a series of tubular prolongations round the 

 filaments of the olfactory nerves as they pass through the cribriform plate, and 

 also round the nasal nerve as it passes through the nasal slit ; a prolongation is 

 also continued through the sphenoidal fissure into the orbit, and another is con- 

 tinued into the same cavity through the optic foramen, forming a sheath for the 

 optic nerve, which is continued as far as the eyeball. In the posterior fossa it 

 sends a process down the internal auditory meatus, ensheathing the facial and 

 auditory nerves; another through the jugular foramen, forming a sheath for the 

 structures which pass through this opening ; and a third through the anterior 

 condyloid foramen. Around the margin of the foramen magnum it is closely 

 adherent to the bone, and is continuous with the dura mater lining the spinal 

 canal. In certain situations, as already mentioned (page 650), the fibrous layers 

 of this membrane separate, to form sinuses for the passage of venous blood. 

 Upon the outer surface of the dura mater, in the situation of the longitudinal 

 sinus, may be seen numerous small whitish bodies, the glandulce Pacchioni. 



Structure. The dura mater consists of white fibrous and elastic tissues 

 arranged in flattened laminae, which are divisible into two layers, the fibres of the 

 two layers intersecting each other obliquely. A layer of nucleated endothelial 

 cells, similar to those found on serous membranes, lines its inner surface ; these 

 were formerly regarded as belonging to the arachnoid membrane. 



Its arteries are very numerous, but are chiefly distributed to the bones. Those 

 found in the anterior fossa are the anterior meningeal branches of the anterior and 

 posterior ethmoidal and internal carotid, and a branch from the middle meningeal. 

 In the middle fossa are the middle and small meningeal branches of the internal 

 maxillary, a branch from the ascending pharyngeal, which enters the skull through 

 the foramen lacerum medium basis cranii, branches from the internal carotid, and 

 a recurrent branch from the lachrymal. In the posterior fossa are meningeal 

 branches from the occipital, one of which enters the skull through the jugular 

 foramen, and the other through the mastoid foramen ; the posterior meningeal, 

 from the vertebral ; occasionally meningeal branches from the ascending pharyngeal, 

 which enter the skull, one at the jugular foramen, the other at the anterior 

 eondyloid foramen, and a branch from the middle meningeal. 



The veins, which return the blood from the dura mater, and partly from the 

 bones, anastomose with the diploic veins. These vessels terminate in the various 

 sinuses, with the exception of two which accompany the middle meningeal artery, 

 and pass out of the skull at the foramen spinosum to join the internal maxillary 

 vein. 



The nerves of the dura mater are, the recurrent branch of the fourth and 

 filaments from the Gasserian ganglion, from the ophthalmic and hypoglossal nerves, 

 and from the sympathetic. 



The so-called glandulee Pacchioni are numerous small whitish granulations, 

 usually collected into clusters of variable size, which are found in the following 



