NERVOUS TISSUE. 



II 



B 



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- 



lae ; 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 meduUated 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 

 non-medullated) bound together by connective 

 tissue, while the bodies of the cells are absent. 

 These nerves are but conducting trunks, bear- 

 ing impulses to or from the central portion of 

 the cell. From their color, those parts which 

 are formed entirely of nerve fibres are called 

 the white matter. The cell bodies, together 

 with fibres, dendrites, etc., unite to form the 

 gray matter, which may be aggregated in 

 smaller centres (ganglia) or in larger continu- 

 ous tracts, as in the brain and spinal cord. 

 In these parts occur certain supporting cells 



Fig. io. Por- 

 tions of medullated 

 nerve fibres (from 

 Martin). The 

 medullary sheath, 

 stained black by 

 osmic acid, is inter- 

 rupted at Ji, the 

 nodes of Ranvier, 

 across which the 

 axis cylinder ex- 

 tends. Outside the 

 medullary sheath is 

 the (mesenchyme) 

 sheath of Schwann, 

 the nuclei of which 

 are seen at c. 



