THE NERVOUS SYSTEM. Ill 



of the first kind, are either nerves of sensation or of motion. 

 These nerves, as discovered by Sir Charles Bell, are entirely 

 distinct, the one rarely, if ever, performing the function of 

 the other. Although these nerves are endowed with different 

 properties, the common function of both is, to give us correct 

 intelligence of the relative condition of our existence. 

 When these functions are interrupted, as in apoplexy, or 

 concussion of the brain, [the individual is deprived of all feel- 

 ing and sensibility, hence they are called the nervous depart- 

 ment of external or animal life.) The nervous filaments 

 which furnish the power of mptionJfarise from the anterior 

 column of the spinal marrowj ^diile those in which sensation 

 resides, are connected with the posterior column.) 



44 The brain fills the~cavity of the cranium or skull, and 

 in an adult weighs between three and four pounds. Its 

 greatest length is about six inches, and its breadth five 

 inches, though its dimensions vary greatly in different per- 

 sons. In infancy its texture is soft, but it grows harder in 

 proportion to the age.^|rThe brain is chiefly composed of 

 two distinct parts, the cerebrum, or brain proper ; and the 

 cerebellum, or the little brain. These again are divided, 

 each into two distinct lobes^ The medulla oblongata is the 

 commencement of the spinal marrow. 



5. fThe brain is surrounded by three membranes ; the two 

 inner ones thin and delicate ; the outer one thick and firm, 

 and called dura mater} It is a perpendicular fold of the dura 

 mater, and divides the brain into two hemispheres. It ad- 

 heres loosely to the bones of the skull ; the internal perios- 

 teum of which it forms. 



6. The other two thin membranes are called arachnoid and 

 pia mater, ffhe first is a serous membrane, as the dura mater 

 isnjibrous membrane ; and forms a completely closed sac or 

 bag. This membrane, which derives its name from its 

 resemblance to a spider's web, spreads over the whole surface 

 of the brain, without dipping into any of its furrows or con- 



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