286 



THE HUMAN BODY. 



of the cerebro-spinal centre, called the arachnoid. In the 

 space between the two layers of the arachnoid is a small 

 quantity of watery cerebro-spinal liquid. 



The Spinal Cord (Fig. 80) is nearly cylindrical in form, 

 being, however, a little wider from side to side than dorso- 

 ventrally, and tapering off at its posterior end. Its aver- 

 age diameter is about f inch and its length 17 inches. 

 It weighs 1-J- ounces. There is no marked limit be- 



FIG. 81. The spinal cord and nerve-roots. A, a small portion of the cord 

 seen from the ventral side; 5, the same seen laterally; C, a cross-section of the 

 eord; D, the two roots of a spinal nerve; 1, anterior (ventral) fissure; 2, poste- 

 rior (dorsal) fissure ; 3, surface groove along the line of attachment of the 

 anterior nerve-roots; 4, line of origin of the posterior roots; 5, anterior root 

 filaments of a spinal nerve; 6, posterior root filaments; 6', ganglion of the pos- 

 terior root ; 7, 7', the first two divisions of the nerve-trunk after its formation 

 by the union of the two roots. 



What is the arachnoid? What is the cerebro-spinal liquid? 

 What is the general form of the spinal cord? Its average diame- 

 ter? Its length? Its weight? How does it connect with the brain? 



