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LESSON XXXI. 

 THE BRAIN. 



THE brain the great nerve centre of the body is the large 

 upper portion of the cerebro-spinal axis filling the cavity of the 

 cranium. It consists chiefly of soft nerve-substance, the general 

 appearance and character resembling that of a sheep or rabbit. 



The average weight of the adult human brain is about three 

 pounds, but the size seems to vary very considerably. 



Fig. 164. The Human Brain. 



A, cerebrum ; B, cerebellum ; c, pons Varolii ; D, medulla oblongata. 

 The parts are represented as separated from one another somewhat 

 more than is natural so as to show their relation better. 



The surface of the brain is covered with a very thin and 

 delicate membrane called the pia mater (Lat. pious mother), which 

 consists of a thickly meshed network of small arteries and veins, 

 supported by connective tissue. It is from this membrane that 

 the brain receives its supply of blood, and consequently it contains 

 none but very small blood-vessels. Outside the pia mater is a 

 delicate and transparent serous membrane (see page 148) called 

 the arachnoid membrane (Gr. arachne, a spider's web ; and eidos, 

 form). Over this is a third membrane, the dura mater (Lat. hard 



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