438 THE TISSUES. 



I. STRUCTURE OF THE NERVES. 

 The nerves contain the following histological elements: 



1. Nerve-fibres. 



2. Areolar tissue. 



3. Bloodvessels. 



1. The nerve-fibres have been sufficiently described (pp. 423-9). 



2. The areolar tissue is insinuated between the individual fibres 

 in the nerve-trunks, and also invests the whole nerve externally, 



and is therefore termed the perineu- 

 Fig. 281. rium. 1 Fig. 281 shows a section of the 



ischiatic nerve of the calf, and the areo- 

 lar tissue between its component nerve- 

 fibres; the latter being first collected 

 into the primary fasciculi, and these 

 / x ^J^HP^c' into the secondary, as has been shown 



. in respect to the fibres of muscle and 



Transverse section of the ischiatic of tendon. The finest Subdivisions (in- 

 nerve of the calf, a External perineu- ternal per i neur i um ) o f fo Q perineurium, 

 rium. b, c. Internal penneurium in- 

 vesting the separate fasciculi of nerve- lying between the fibres are in the form 



rrCSS ?%% <* * homogeneous membrane, with nu- 



clei 4^3 of an inch in diameter, and 



which may be regarded as embryonic collagenous tissue (p. 276). 

 The elastic fibres are not seldom entwined around whole fasciculi. 



3. The bloodvessels of the nerves are not very numerous. They 

 extend principally in a longitudinal direction, and form a loose 

 plexus of minute capillaries of g^ 1 ^ to 3^^ of an inch, with elon- 

 gated interstices. This invests the fasciculi, and penetrates them, 

 but never surrounds individual nerve-fibres. 



The nerves, in respect to their origin, are of three kinds, and 

 will be described in the following order: 



1. The spinal nerves. 



2. The ganglionic nerves. 



3. The encephalic nerves. 



1 From nipt, around, and vsfyov, a nerve. 



