126 GENERAL FEATURES OF NERVOUS TISSUES. 



under a higher power, either after having been isolated or in an adequately 

 thin and prepared section, will present the following features : 



The cell consists of a cell body which is, normally, pear-shaped, having a 

 broad end in which is placed the nucleus, and a narrow end which thins out 

 into a stalk, and is eventually continued on as a nerve fibre. The substance 

 of the cell body is of the kind which we call finely granular protoplasm ; 

 sometimes there is an appearance of fibrillation, the fibrillse passing in 

 various directions in the body of the cell and being gathered together in a 

 longitudinal direction in the stalk. Sometimes the cell body immediately 

 around the nucleus appears of a different grain from that nearer the stalk, 

 and not unfrequently near the nucleus is an aggregation of discrete pigment 

 granules imbedded in the protoplasm. 



The nucleus, like the nuclei of nearly all nerve cells, is large and con- 

 spicuous, and when in a normal condition is remarkably clear and refractive, 

 though it appears to consist like other nuclei of a nuclear membrane and 

 network and nuclear interstitial material. Even more conspicuous, perhaps, 

 is a very large spherical, highly refractive nucleolus; occasionally more than 

 one nucleolus is present. 



Surrounding the cell body is a distinct sheath or capsule consisting of a 

 transparent, hyaline, or faintly fibrillated membrane, lined on the inside by 

 one layer or by two layers of flat, polygonal, nucleated epithelioid cells or 

 plates ; that is to say, cells which resemble epithelium cells, but differ not 

 only in being extremely flattened, but also in the cell body being transformed 

 from ordinary granular protoplasm into a more transparent differentiated 

 material. In stained specimens the nuclei of these plates are very conspicu- 

 ous. Under normal conditions this sheath is in close contact with the whole 

 body of the cell, but in hardened and prepared specimens the cell body is 

 sometimes seen shrunk away from the sheath, leaving a space between them. 

 Occasionally the cell body while remaining attached to the sheath at three 

 or four or more points is retracted elsewhere, and accordingly assumes a 

 more or less stellate form ; but this artificial condition must not be con- 

 founded with the natural branched form which, as we shall see, other kinds 

 of nerve cells possess. 



When a section is made through a hardened ganglion, the plane of the 

 section passes through the stalks of few only of the cells, and that rarely for 

 any great distance along the stalk, since in the case of many of the cells the 

 stalk is more or less curved and consequently runs out of the plane of sec- 

 tion ; but in properly isolated cells we can see that in many cases, and we 

 have reasons to believe that in all cases, the stalk of the cell is, as we have 

 said, continued on into a nerve fibre. As the cell body narrows into the 

 stalk several nuclei make their appearance, lodged on it ; these are small 

 granular nuclei, wholly unlike the nucleus of the cell body itself, and more 

 like, though not quite like, the nuclei of the neurilemma of a nerve. They 

 are probably of the same nature as the latter ; and, indeed, as we trace the 

 narrowing stalk downward a fine delicate sheath, which, if present, is at 

 least not obvious over the cell body, makes its appearance, and a little further 

 on, between this sheath, which is now clearly a neurilemma, and the stalk of 

 the cell body, which has by this time become a cylinder of uniform width 

 and is now obviously an axis-cylinder, a layer of medulla, very fine at first, 

 but rapidly thickening, is established. The stalk of the nerve cell thus 

 becomes an ordinary medullated nerve fibre. The sheath of the cell is con- 

 tinued also on to the nerve fibre, not as was once thought as the neurilemma, 

 but as that special sheath of connective tissue of which we have already 

 spoken as Henle's sheath, and which ultimately becomes fused with the con- 

 nective tissue of the nerve. 



