SPINAL CORD AND NERVES 



533 



is the dura mater, which is fibrous and vascular ; inside this are 

 the arachnoid, and then the pia mater, consisting of flattened 

 cells. Between these two, and in places between the arachnoid 



Fig. 416. — A diagram of fibres entering and leaving the spinal cord, showing 

 various tracks along which impulses may be conducted in the ' exchange ' 

 system which it constitutes. The arrows show the direction of impulses. 



Note that where the terminal branches of the axon of one neurone meet the 

 dendrites of another the two are not continuous but interlace, so that the 

 nervous impulse must pass an interruption in its track. This arrangement is 

 called a synapse. 



I. The simplest track, one afferent and one efferent neurone ; 2. an intermediate neurone is concerned; 



3. the track crosses the cord, so that an impulse is discharged along a nerve on the opposite side ; 



4. a neurone receives impulses from two others 15,5', 5", a fibre branches to affect neurones in different 

 parts of the cord ; d.r.g.c, cells of dorsal root ganglion. 



and dura mater, are spaces containing cerebrospinal fluid, 

 which is secreted in the ventricles of the brain and returns to the 

 venous system. The nervous 



tissue is divided into an outer 

 white matter, consisting 

 mainly of nerve fibres, and an 

 inner, somewhat X-shaped 

 grey matter, consisting mainly 

 of cell bodies. In the very 

 centre is a small central canal. 

 The brain is generally similar, 

 with, in the hemispheres, a 

 great development of the grey 

 matter. Nerves (Fig. 417) are 

 bundles of nerve fibres bound 

 together by connective tissue. 





t. 



i?. 



ar. 



Fig 



417. — A transverse section 

 medullated nerve of the 

 stained with osmic acid 

 magnitied. 



•/ 



of a 



frog, 

 and 



ar., .•\rtcry ; c.t., connective-tissue sheath or peri- 

 neurium ; /., funiculus or bundle of nerve 

 fibres : v. vein. 



