496 NERVOUS SYSTEM. 



than the brain ; and, according to L'Heritier, the nerves contain more albumen 

 and more soft fat than the brain. 



Microscopic Structure. The fibrous nervous matter consists of two different 

 kinds of nerve fibres, which are distinguished as the tubular fibre and the gela- 

 tinous fibre. In most nerves these two kinds are intermingled; the tubular 

 fibres being more numerous in the nerves of the cerebro-spinal system, the gela- 

 tinous predominating in the nerves of the sympathetic system. 



The tubular fibres appear to consist of tubules of simple membrane, homogeneous 

 in structure, and analogous to the sarcolemma of striped muscle. Within is the 

 proper nerve substance, composed apparently of two different materials ; the central 

 part, which occupies the axis of the nerve tube, is a transparent material, termed 

 the axis cylinder ; while the outer portion, which forms a tube within the tubular 

 membrane, inclosing the axis-cylinder, is usually opaque and dimly granular, as 

 if from a kind of coagulation, and is generally known as the white substance 

 of Schwann. The peculiar white appearance of the cerebro-spinal nerves is 

 principally due to the large amount of the white substance of Schwann which they 

 contain. It is probable that the essential element of the nerve tube is the axis 

 cylinder, the tubular membrane and white substance of Schwann affording me- 

 chanical protection to this substance, insulating it from the constituent parts of 

 the neighboring fibres. 



In a perfectly fresh state, the nerve tubes present the appearance of simple 

 membranous tubes, perfectly cylindrical, and containing a transparent and ap- 

 parently homogeneous material ; but shortly after death, when pressed or separated 

 by coarse manipulation, they undergo changes which render it probable that their 

 contents are composed of the two materials above mentioned, for the fine outline 

 of the previously cylindrical tube is exchanged for a dark double contour, the 

 outer line being formed by the tubular sheath, the inner by the white substance of 

 Schwann, at the same time the granular material collects into small masses w r hich 

 distend the tubular membrane at irregular intervals, while the intermediate spaces 

 collapse, giving the fibres a varicose or beaded appearance. In the brain, spinal 

 cord, and nerves of special sense, the tubes are very apt to exhibit this change, 

 owing to extreme thinness of the tubular membrane and to a diminished con- 

 sistence of the contained nervous matter. The contents of the nerve tubes are 

 extremely soft, for when subjected to slight pressure they readily pass from one 

 part of the canal to another, and often cause a bulging at the side of the tube. 

 The contents, also, readily escape on pressure from the extremities of the tube, 

 assuming the appearance and form of globules, consisting of a transparent central 

 part, surrounded by a layer of the white substance of Schwann, marked by its 

 double contour. 



The nerve fibres vary in size ; they are largest within the trunk and branches 

 of the nerves, measuring from ^V^r to -J^TT of an inch. They become gradually 

 smaller as they approach the brain and spinal cord, and usually also in the tissues 

 in which they are distributed. In the gray matter of the brain and spinal cord, 

 they seldom measure more than y^^^^ to y^^c of an inch. 



The gelatinous fibres constitute the main part of the trunk and branches 

 of the sympathetic nerve, and are intermingled in various proportions in the 

 cerebro-spinal nerves. "When collected together in great numbers, they exhibit 

 a yellowish -gray color. They are flattened, soft, and homogeneous in appearance, 

 containing nuclei, of a round or oval form, arranged at nearly equal distances, 

 and frequently presenting nucleoli. They vary in diameter, from g^^ to ^^ 

 of an inch, and resemble much the fibres of unstriped muscle. They differ from 

 the tubular fibres in their smaller size, being only one-half or one-third their size, 

 in the absence of the double contour, their apparently uniform structure, and their 

 yellowish-gray color. It appears probable that these nerves are composed ex- 

 clusively of the substance which corresponds with the axis cylinder of the tubular 

 nerves, and differs from them in not possessing the tubular membrane, and white 

 substance of Schwann. 



