NERVOUS TISSUE. 



II 



length, and is known as an axis cylinder or axon ; the others 

 are shorter, and as they soon break up into 

 minute branches, they are called dendrites. 

 In most cases the axis cylinders have a similar 

 method of termination. Recent investigations 

 show that the only connection between nerve 

 cells consists in an interlacing of these fibril- 

 Ice ; two nerve cells never join. 



The axis cylinder is the essential part of 

 a nerve fibre. Of these fibres two kinds are 

 to be recognized. In the medullated fibres 

 the axis cylinder is surrounded by a medul- 

 lary sheath of a peculiar substance (myelin) 

 rich in fat. This sheath, it is to be noted, 

 usually stops before the end of the axis cylin- 

 der, and in most cases it is not continued to 

 the central mass of the cell. In the non-medul- 

 lated fibres the sheath is lacking, and only the 

 axis cylinder is found. Both medullated and 

 non-medullated fibres may have a second sheath 

 (the neurilemma or sheath of Schwann) de- 

 rived from the connective tissue (see below), 

 and containing scattered nuclei. 



Nervous tissue is made up of these nerve 

 cells. In a nerve proper we have but a bundle 

 of nerve fibres (axis cylinders, medullated or FIG. 10. Por- 

 non-medullated) bound together by connective tions of medulla ted 



... - nerve fibres (from 



tissue, while the bodies of the cells are absent. Martin). The 

 These nerves are but conducting trunks, bear- medullary sheath, 

 ing impulses to or from the central portion of stamed black b Y 



. ,, . ... osmic acid, is inter- 



the cell. From their color, those parts which rupted at R ^ tne 

 are formed entirely of nerve fibres are called nodes of Ranvier, 

 the white matter. The cell bodies, together across which the 



.., ~, . . . - . axis cylinder ex- 



with fibres, dendrites, etc., unite to form the tends> Outside the 

 gray matter, which may be aggregated in medullary sheath is 

 smaller centres (ganglia) or in larger continu- the Oesenchyme) 



, , 1-1 i sheath of Schwann, 



ous tracts, as in the brain and spinal cord. the nudei of which 

 In these parts occur certain supporting cells are seen at c. 



