THE NERVOUS SYSTEM 2OQ 



Having described briefly the several portions that go 

 to make up the nervous system, we will now consider 

 more fully the anatomy of the parts connected with these 

 important structures. 



THE BRAIN 



This is that portion of the cerebrospinal center con- 

 tained in the cavity of the cranium. It is divided into 

 four principal parts : the medulla oblongata, pons Varolii, 

 cerebrum, and cerebellum. 



The average weight of the brain is about 49 ounces in 

 the male and 44 ounces in the female; the heaviest male 

 brain of which there is a record weighed 68 f ounces; the 

 lightest, 34 ounces; the heaviest female brain weighed 56 

 ounces; the lightest, 30 ounces. 



The brain is divided into fissures and lobes ; the fissures 

 divide the several parts of the brain substance, and in 

 this manner the several lobes are marked out. 



As has been stated elsewhere, the brain is surrounded 

 by three membranes: the dura mater, which is a tough, 

 fibrous membrane ; the arachnoid, a serous membrane ; and 

 the pia mater, a vascular membrane. 



The Dura Mater. This is the first membrane to be 

 encountered after the removal of the skull-cap. It 

 derived its name from the fancied belief that it supplied 

 all the membranes of the body. As has been said, it is a 

 tough, inelastic, fibrous structure, forming the internal 

 periosteum of the skull. It acts as a support for the 

 lobes of the brain, and forms partitions for the falx cerebri, 

 falx cerebelli, and tentorium cerebelli; it also provides 

 sinuses for the veins that return the blood from the brain ; 

 further, it forms sheaths for the nerves as they leave the 

 skull. 



The Arachnoid Membrane. This is the second mem- 

 brane, and takes its name from its resemblance to a spider's 



14 



