68 



NORMAL HISTOLOGY. 



cortex, and especially those constituting the chief 

 layer of the cerebellum, are distinguished by 

 cell-bodies and the peculiar ramifications and 

 their dendrites. Within the cere- 

 found examples of multipolar neu- B 

 axones almost immediately undergo 

 gray matter to which they are con- 

 The Nerve-Fibres. From 

 above, it is evident that nerve-fibres 



components of the granule 

 the small size of their 

 claw-like telodendria of 

 bellar cortex are also 

 rones of type II, whose 

 branching within the 

 fined. 



what has been said 

 are not independent ele- 



Axis-cylinders 



FIG. 90. Multipolar nerve cells of various forms ; A, from spinal cord ; B, from cerebral cortex ; C, from 

 cerebellar cortex ; a, axone ; c, implantation cone. 



ments, but only the processes of neurones either the axones that are pro- 

 longed as fibres, or the dendrites of neurones situated within the spinal and 

 other sensory ganglia. Although neurones exist which are not continued 

 as nerve-fibres, the converse is not true, since nerve- fibres are always 

 connected with neurones. 



The fundamental part of every nerve-fibre is the central cord, known as 

 the axis-cylinder, which is composed of delicate threads, the axis-fibrilla , 



prolonged from the nerve-cell and embedded 

 within a semifluid interfibrillar substance, the 

 neuroplasm. In the case of the typical fibres, 

 such as form the chief constituents of the periph- 

 eral nerves, the axis-cylinder is surrounded by 

 a relatively thick coat, known as the medullary 

 sheath, outside of which lies a thin structureless 

 envelope, the neurilemma or sheath of Schwann. 

 These coverings, however, do not invest the 

 entire nerve. Thus, for a short distance after 

 leaving the nerve-cell, the axis-cylinder is with- 

 out covering ; soon it becomes surrounded by 

 the medullary sheath, and then, if it be a 

 peripheral nerve, acquires the outer envelope, 

 the neurilemma. In the case of the nerve- 

 fibres that course within the brain and spinal 

 cord, the fibre is devoid of the neurilemma, 

 although it usually possesses the medullary coat. 



The medullary sheath consists of two parts, a delicate reticular 

 framework and a fatty substance, the myelin, that fills the meshes of the 



Neurilemma 



FIG. 91. Medullated nerve-fibres, 

 as seen in longitudinal section of 

 spinal nerve. X 375. 



