. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. 740 TEE NERVOUS SYSTEM IN GENERAL. 407. riAGRAM OF STRUC- TURE OF NERVE- FIBRE. 1, Sheath; 2, medul- lary substance of Schwann ; 3, axis- cylinder, or primi- tive band. Fig. 408. brane, within wliich is a layer of protoplasm containing a nucleus. The axis of the segment is occupied by a slender cylindrical stalk {axis-cylinder of Fiirkinje, or band of'Remak), the space between it and the protoplasm being filled by a viscid opaque substance {myelin, medtdht, white substance of Schwann, or sheath of the marrovf),
. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. 740 TEE NERVOUS SYSTEM IN GENERAL. 407. riAGRAM OF STRUC- TURE OF NERVE- FIBRE. 1, Sheath; 2, medul- lary substance of Schwann ; 3, axis- cylinder, or primi- tive band. Fig. 408. brane, within wliich is a layer of protoplasm containing a nucleus. The axis of the segment is occupied by a slender cylindrical stalk {axis-cylinder of Fiirkinje, or band of'Remak), the space between it and the protoplasm being filled by a viscid opaque substance {myelin, medtdht, white substance of Schwann, or sheath of the marrovf), whi(;h i8 stained black by osmic acid, and is quickly coagulated by cold. The axis-cylinder is not interrupted at the points of con- tact of the segments, but passes through the annular con- strictions, and runs from beginning to end of the nerve. When the medulla is solidified, it is seen to be bordered by two dark lines, parallel to the walls of the nerve-tubes ; this aspect has caused the latter to be named '• double-con- toured tubes (or nerve-fibres)." All the nerve-tubes do not possess, at the same time, these three parts, for the medulla may be absent; so that there are distinguished medullated and non-medullated nerve- fibres. The first, more or less thick, are met with in the nerve-centres, and at the origin and middle portion of the nerves ; the second are found at the termi- nation of nerves, and in the great sympathetic. There are also observed in the nerves of organic life, elongated elements, designated ^ftres of Renmk,grey fibres, and nucleated nervous fibres. These are pale, flattened fibres, with parallel borders, and furnished with elliptical nuclei. Some authorities consider these to be bands of connective tissue, and not nerve elements. The nerve-cells, or corpuscles, are voluminous, and formed by a mass of protoplasm without any enveloping membrane, but in the midst of which are numerous fibrillar. In the ganglia they are covered by a layer of fibril
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