MENINGES OF THE BRAIN. 469 



it blends with the periosteum in the first bones of the coccyx. 

 Excepting near the posterior part, where they are attached by 

 tibrous slips, the dura mater is separated from the periosteum of 

 the canal by areolar tissue and a plexus of veins, so that it forms 

 a loose sheath round the cord. Its inner surface is smooth, and 

 lined by the arachnoid membrane ; opposite each intervertebral 

 foramen it is pierced for the passage of the spinal nerves, which 

 it accompanies, as a tubular sheath, through the foramen. 



The arachnoid (so called from its extreme tenuity, resembling 

 somewhat a spider's web) is a delicate serous membrane, and, like 

 other serous membranes, is a shut sac. Its visceral layer loosely 

 envelops the pia mater of the cord, leaving between them an 

 interval, the suh-arachnoidean space, continuous with an opening 

 at the posterior border of the brain, which contains the limpid 

 cerebro-spinal fluid. The parietal layer is a continuation of the 

 visceral, the latter surrounding the nerves as they leave the cord 

 until they pierce the dura mater, when it is reflected on to the 

 inner surface of that membrane. 



The pia mater, the inner envelope of the cord, is a thin vascular 

 membrane, composed of areolar tissue, containing blood-vessels. 

 It is less vascular, but thicker, denser, and more fibrous than the 

 corresponding membrane of the brain, of which it is a continua- 

 tion. It closely invests the whole surface of the cord, sends pro- 

 cesses into its longitudinal fissures, and forms a sheath for the 

 spinal nerves. When the cord terminates, the pia mater is pro- 

 longed as a slender filament {filum termincde) through the 

 centre of the mass of nerves forming the caiida equina, and 

 blends with the terminal cord of the dura mater. 



The ligamentimi denticulatuTn is a narrow fibrous band, 

 which runs between the superior and inferior roots of the spinal 

 nerves, from the foramen magnum backwards, so as to form a 

 support for the cord. Its inner border is continuous with the 

 pia mater ; the denticulations of its outer edge being attached to 

 the dura mater in the intervals between the roots of the nerves. 



MENINGES OF THE BRAIN. 



The Dura mater is the thick, white, fibrous membrane which 

 lines the cavity of the cranium, acting as the internal periosteum 

 of the bones. It is continuous with the spinal dura mater at the 

 foramen magnum, and is also prolonged as a sheath for the nerves. 



