CH. XLIV.] 



THE CENTRAL NERVOUS SYSTEM 



643 



connective-tissue membranes, known from without inwards as dura 

 mater, arachnoid, and pia mater respectively. 



In Chapter XVI. we have already considered some of the 

 elementary and fundamental problems in relation to the activities of 

 nerve centres, and it would be at this point advisable that the 

 student should refresh his memory on such points by again reading 

 that chapter before he proceeds further. 



The next few chapters will deal with that portion of the anatomy 

 of the spinal cord and brain which one must know before it is possible 

 to study profitably the functions of these parts, and we shall start 

 with the spinal cord and reach the cerebrum last. 



Before, however, passing on to these details, a few general words 

 are necessary in relation to the construction of the central nervous 

 system in vertebrate animals. 



A student's first glance at the human brain, or at such a drawing 

 of it as is given in the accompanying figure (fig. 395), will be sufficient 

 to convince him of its complicated structure. The next figure, how- 

 ever, representing semi-diagrammatically its different parts, will make 

 an enumeration of its subdivisions more intelligible. 



At the lowest part of the brain (fig. 396), continuing the spinal 

 cord upwards, is the medulla oblongata or bulb (D). Next comes the 



FIG. 396. Plan in outline of the brain, as seen from the right side. . The parts are represented as 

 separated from one another somewhat more than natural, so as to show their connections. A, 

 cerebrum; /, g, h, its anterior, middle, and posterior lobes; e, fissure of Sylvius; B, cerebellum ; 

 C, pons Varolii ; D, medulla oblongata ; a, peduncles of the cerebrum ; b, c, d, superior, middle, and 

 inferior peduncles of the cerebellum. (From Quain.) 



pons Varolii (C), very appropriately called the bridge, because in it 

 are the connections between the bulb and the upper regions of the 



