NERVvVOusS TISSUES. 207 
dead animal the arteries appear white, flat or collapsed, and empty. 
The veins on the other hand appear large and dark on account of 
their distension with blood. 
4. Nervous Tissues. 
Nervous tissues form the basis of the central nervous system 
and of the outlying nerves and ganglia. They comprise two kinds 
of elements—nerve cells and nerve fibres. In the central nervous 
system these elements are imbedded in a mass of neutral tissue, the 
neuroglia. 
Nerve cells are characteristic of the central 
nervous system and of the spina! and sympathe- 
tic ganglia. They differ greatly in form, but 
typically each consists of a cell-body (Fig. 17) 
bearing two kinds of processes—a fibre-process, 
the neuraxis or neurite, and a series of branch- 
ed protoplasmic processes, the dendrites. The 
cell-body is distinguished by the presence in its 
interior of granular masses, the chromatophile 
or tigroid bodies. The latter extend into the 
dendrites, but not into the neuraxis. The 
dendrites may be greatly elaborated, and may be Pee che PerD samt ot 
present to a considerable number. The neuraxis two, medullated nerve- 
fibres from the sciatic 
is a nerve fibre process. Since it continues as of the rabbit: a.c., axial 
cord; m.s., myelin 
the central portion or axial cord of a nerve fibre, pices luce oer 
it may traverse a relatively enormous distance _nevzilemma; nr. node 
on its way to a peripheral organ. 
A nerve fibre consists of a central core, the axial cord, enclosed, 
except in the case of those of the olfactory nerve, by certain mem- 
branes. Two kinds of fibres are distinguished—medullated fibres, 
and non-medullated fibres. The former are characteristic of 
the peripheral nerves. In these (Fig. 18) the axial cord is sur- 
rounded by a comparatively thick membrane of fatty material, the 
medullary or myelin sheath. The latter is continuous except 
at certain points, the nodes of Ranvier, where the axial cord 
appears free except for an external investment of the whole fibre, 
the neurilemma. 

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