324 



HITCHCOCK'S ANATOMY 



branes envelop the brain, although to the unassisted eye they 

 appear as one. They are of great importance in the economy 

 of the brain, and are often the seat of severe disease. The 

 outer one is called the Dura Mater, that covers both the 

 brain and spinal cord, and is attached firmly to the bones 

 which it covers. It has the same composition as the liga- 

 ments of the body — white fibrous tissue — and consequently is 

 very tough, being the firmest of the three membranes. 



576. Arachnoid Membrane —The Arachnoid Membrane 

 lies directly beneath the dura mater, and receives its name 

 from the Greek word signifying spider's web. 



577. Pia Mater.— Closely beneath it is the Pia Mater, 

 which lies directly upon the surface of the brain, and dips 

 into all the cavities or convolutions on its surface. This 

 membrane is especially serviceable in the nourishment of the 

 brain, and receives the arteries which enter at the anterior 

 and posterior part of the skull. Its nerves are branches of 

 the sympathetic system. 



578. Surface of the Brain. — The surface of the brain is 

 very uneven, being covered with convolutions or tortuous 

 ridges and corresponding depressions, the design of which is 

 not as yet well known, though probably merely to procure a 

 greater amount of surface, and they do not correspond to the 

 irregularities on the surface of the skull from which phre- 

 nologists profess to judge of character. 



579. Ventricles of the Brain. — Within the brain are sev- 

 eral cavities called ventricles, of which the special use has not 

 been determined. (Fig. 305, p. 320.) It is a curious fact, 

 however, that a post mortem examination of the brain of in- 

 ebriates frequently discovers these cavities to be partially filled 

 with alcohol. 



580. Division of the Nerves. — All the nerves that pro- 



575. How many membranes enveloping the brain? Describe the dura mater. 576. 

 Describe the arachnoid membrane. 577. Describe the pia mater. What are its nerves? 

 573. What is the condition of the surface of the brain ? 579. What are the cavities in the 

 brain called? 580. How are all the nerves given off from the brain and spinal cord ? 



