94 COMPARATIVE ANATOMY 



axial fibre, and the neurilemma alone from the nucleated 

 sheath of Schwann ; and the phenomena of regenerating nerve 

 fibres certainly lend support to their view. The subject is com- 

 plicated, and cannot- be profitably discussed in this place ; but 

 it is important to remember, firstly,- that the axis fibre is an 

 outgrowth from, and is nourished by, a nerve-cell ; secondly, 

 that the nodes of Rarivier mark off the limits of cells which 

 have become secondarily related to the axis fibre, and are 

 represented by the nuclei found in each internode. It may 

 remain an open question whether the cells in question form 

 both medullary sheath and neurilemma or the latter only. 



A further peculiarity may be mentioned, that in most pre- 

 parations the medullary sheath is seen .to be broken up into 

 segments, with conical or funnel-shaped ends which fit into 

 one another. There may be many such in a single internode, 

 and it is a moot point whether they exist in the living nerve or 

 whether they are the result of post-mortem changes, induced 

 perhaps by the reagents used in the preservation of the nerves. 

 Towards their terminations, both afferent and efferent nerves 

 lose their medullary sheath, and the axis cylinder breaks up 

 into minute ramifying fibrils which exhibit characteristic 

 varicosities. These terminal ramifications are distributed in 

 various ways ; as, for instance, over the surface of a muscle in 

 the case of efferent nerves, or a fibril may be continuous, as 

 we have seen, with internal process of a sense-cell, in the 

 case of afferent nerves. 



The nervous tissues are the eminently irritable, the muscular 

 tissues the eminently contractile. Muscular tissue is of two 

 kinds, plain or non-striated, and striated or striped muscle. 

 The latter presents two important varieties : the one found in 

 the heart is known as cardiac muscular tissue, the other forms 

 the skeletal muscles. As plain muscle-fibre is usually removed 

 from the control of the will it is also called involuntary, and 

 the striped muscle, being under the control of the will, is called 

 voluntary; but these names are not satisfactory, since cardiac 

 muscle, though striated, is involuntary. 



Plain muscular tissue (fig. 17) is found in the walls of the 

 alimentary tract, in the walls of the arteries and veins, in the 

 bladder, and in other viscera. It may occur in bundles or in 

 layers, and is formed of distinct cellular elements, which may 

 be separated from one another by suitable methods. A plain 



