Nervous Tissues. 



37 



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 spinal 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 

 present to a considerable number. The neuraxis 

 is a nerve fibre process. Since it continues as 

 the central portion or axial cord of a nerve fibre, 

 it may traverse a relatively enormous distance 

 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 — meduUated 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. 



(-_n.c 



Fig. 18. Portions of 

 two meduUated nerve- 

 fibres from the sciatic 

 of the rabbit : a.c, axial 

 cord; m.s., myelin- 

 sheath, stained black 

 with osmic add; n., 

 neurilemma; n.r., node 

 of Ranvier. 



