ANATOMICAL DESCRIPTION 283 



418. Plexuses. We have already seen that in many 

 regions of the body, especially about the neck, loins, and 

 pelvis, adjacent nerves interlace to form a network, or 

 nervous plexus. Nerves passing outward from a plexus 

 contain fibers from several different nerves, and the parts 

 to which these nerves are distributed are thus sometimes 

 brought into connection with a considerable part of the 

 central nervous system. The nerves which supply the 

 limbs, where very complex muscular movements are re- 

 quired, and where nervous coordination of those move- 

 ments is necessary, come from large plexuses where fibers 

 from many nerves are intermingled. (It should be remem- 

 bered that the term " network " is here used subject to the 

 limitation given above [p. 279] and does not imply more 

 than is there stated.) 



By means of the spinal nerves nearly all the nervous 

 impulses from the trunk and limbs pass through the spi- 

 nal cord to the brain. 



419. Membranes of the Brain and Spinal Cord. Three 

 membranes inclose the brain and spinal cord. The dura 

 mater is a tough, white fibrous and elastic membrane lin- 

 ing the bony cavity of the skull which contains the brain, 

 and the vertebral canal in which lies the spinal cord. It 

 also lies in folds between the divisions of the brain. 



The arachnoid is a thin membrane of loose connective 

 tissue, forming a sort of closed sac (like the pericardium), 

 which secretes a serous fluid. It lies between the dura 

 mater and the pia mater. 



The pia mater is an extremely delicate, highly vascular 

 membrane (that is, having many blood vessels), closely 

 investing the nervous matter of the spinal cord and the 

 brain. It follows all the curves and convolutions of the 

 brain, and carries to all its parts and to the spinal cord 



