PHYSIOLOGICAL ANATOMY OF THE NERVOUS TISSUE. 567 



altered by desiccation, the action of water, acetic acid, and various other reagents, it 

 coagulates into an opaque, granular mass. The consistence of this substance gives to the 

 medullated fibres a very peculiar appearance. The tubular membrane being very thin 

 and not elastic, the white substance, by very slight pressure, is made to fill the tubes 

 irregularly, giving them a varicose appearance, which is entirely characteristic. In 

 examining a preparation of the nervous tissue, large drops, coagulated in irregular shapes, 

 are seen scattered over the field and frequently fringing the divided ends of the tubes. 

 In the white substance of the encephalon and spinal cord, where the tubular membrane 

 is wanting, the varicose appearance of the fibres is more remarkable than in any other 

 situation. 



The axis-cylinder is, in all probability, the essential anatomical element of the nerves. 

 It exists in all the nerves except in those termed gelatinous fibres, or fibres of Remak, 

 which will be described hereafter. In the ordinary medullated fibres, the axis-cylinder 

 cannot be seen in the natural condition of the tissue, because it refracts in the same 

 manner as the medullary substance, and it cannot be demonstrated afterward, on account 

 of the opacity of the coagulated matter. If a fresh nerve, however, be treated with 

 strong acetic acid, the divided ends of the fibres will retract, leaving the axis-cylinder, 

 which is but slightly affected by reagents. It then presents itself in the form of a pale, 

 slightly-flattened band, with outlines tolerably regular, though slightly varicose at inter- 

 vals, somewhat granular, and sometimes very finely striated in a longitudinal direction. 

 This band is elastic but not very resisting. Its granules are excessively pale. What 

 serves to distinguish it from all other portions of the nerve-fibre is its insolubility in most 

 of the reagents employed in anatomical investigations. It is slightly swollen by acetic 

 acid but is dissolved after prolonged boiling. If a solution of carmine be applied to the 

 nervous tissue, the axis-cylinder only is colored. It has been remarked that the nerve- 

 fibres treated with nitrate of silver present in the axis-cylinder well-marked transverse 

 striations ; and some observers are disposed to regard both the nerve-cells and the axes 

 of the fibres as composed of two substances, the limits of which are marked by the regu- 

 lar striaa developed by the nitrate of silver. This, however, is a point of purely anatomi- 

 cal interest. The presence of regular and well-marked striae in the axis-cylinder after 

 the addition of a solution of nitrate of silver and the action of light cannot be doubted ; 

 but it has not yet been determined beyond question whether these markings be entirely 

 artificial, or whether the axis-cylinder be really composed of two kinds of substance. 



A still more important question with regard to the intimate structure of the axis- 

 cylinder refers to the longitudinal striations. These are observed in many fibres, but 

 they are not constant. Some authors have adopted the view that the markings are pro- 

 duced by fibrillse, analogous to the fibrilta of the muscular fibres, in all the fibres, as well 

 as in those of the retina, the olfactory, and some of the sympathetic nerves. In the organs 

 of special sense, there can be no doubt of the existence of fibrillse ; but this is by no 

 means so clearly demonstrable in the general system of nerves. Still, it is necessary to 

 take into consideration, in this connection, certain facts with regard to the origin of the 

 nerve-fibres in the cells and their ultimate distribution in sensitive parts. In the final 

 distribution of sensitive nerves, we shall see that the fibres break up into filaments 

 resembling fibrillas ; and, although the fibrillated character of the poles of the nerve-cells 

 is not unreservedly accepted by anatomists, many observers positively state that such is 

 their structure. In the present condition of the science, we cannot do more than state 

 that, while a fibrillated structure has perhaps been shown in the nerves of some of the 

 lower orders of animals, its existence in man and in the mammalia is somewhat doubtful. 



The diameter of the axis-cylinder is about one-half or one-third that of the tube in 

 which it is contained. The various appearances which the nerve-fibres present under 

 different conditions are represented in Fig. 174. 



Simple, or Non -medullated Nerve- Fibres. These fibres are found very largely dis- 

 tributed in the nervous system. When we come to study the structure and relations of 



