530 



STRUCTURE OF NERVE-FIBRES. 



the lymphatics {Axel Key and Retziiis).'] The nerve-fibres within any bundle are held together 

 by delicate connective-tissue, which penetrates between the adjoining fibres, constituting the 

 endoneurium (cd). It consists of delicate fibres with branched connective-tissue corpuscles 

 (fig. 368, 6, d), and in it lie the capillaries, which are not very numerous, and are arranged to 

 form elongated open meshes. 



[Henle's Sheath. When a nerve is traced to its distribution, it branches and becomes smaller, 

 until it may consist only of a few bundles or even a single bundle of nerve-fibres. As the buudle 

 branches, it has to give oft" part of its lamellated sheath or perineurium to each branch, so that, 

 as we pass to the periphery, the smaller bundles are surrounded by few lamellae. In a bundle 

 containing only a few fibres, this sheath may be much reduced, or may consist only of thin, 

 flattened, connective-tissue corpuscles with a few fibres. A sheath surrounding a few nerve- 

 fibres is called Holies Sheath by Ranvier.] 



[Nervi Nervorum. Marshall and v. Horsley have shown that the nerve-sheaths are provided 

 with special nerve-fibres, in virtue of which they are endowed with sensibility.] 



Development. At first nerve-fibres consist only of fibrils, i.e., of axis-cylinders, which 

 become covered with connective substance, and ultimately the white substance of Schwann is 



developed in some of them. The growth in length of the 

 fibres takes place by elongation of the individual " inter- 

 annular " segments, and also by the new formation of these 

 ( Vignal). 



II. Ganglionic or Nerve-Cells. 1. Multipolar nerve-cells 

 (fig. 368, I) occur partly as large cells (100 fi), and are visible 

 to the unaided eye as in the anterior horn of the spinal cord, 

 and in a different form in the cerebellum, and partly in a 

 smaller form (20 to 10 fi) in the posterior horns of the spinal 

 cord, many parts of the cerebrum and cerebellum, and in the 

 retina. They may be spherical, ovoid, pyramidal [cerebrum], 

 pear- or flask-shaped [cerebellum], and are provided with 

 numerous branched processes which give the cells a char- 

 acteristic appearance. [Deiters isolated such cells from the 

 anterior horn of the grey matter of the spinal cord, so that 

 this special form of cell is sometimes called "Deiters' cell" 

 (fig. 368, I).] They are devoid of a cell envelope, are of soft 

 consistence, and exhibit a fibrillated structure, which may 

 extend even into the processes. Fine granules lie scattered 

 throughout the cell-substance between the fibrils. Not unfre- 

 quently yellow or brown granules of pigment are also found, 

 either collected at certain parts in the cell or scattered through- 

 out it. The relatively large nucleus consists of a clear envelope 

 enclosing a resistant substance. It does not appear to have 

 a membrane in youth (Schivalbe). Within the nucleus lies the 

 nucleolus, which in the recent condition is angular, provided 

 with processes and capable of motion, but after death is highly 

 refractive and spherical. There is always one unbranched 

 process, constituting the axial cylinder process (I, z) which 

 remains unbranched, but it soon becomes covered with the 

 white substance of Schwann, and the other sheaths of a 

 mcdullated nerve, so that it becomes the axial cylinder of a 

 nerve-fibre. [Thus a nerve-fibre is merely an excessively long, 

 unbranched process of a nerve-cell pushed outwards towards 

 the periphery. ] It is not definitely ascertained that the cerebral 

 cells have such processes. All the other processes divide very 

 frequently until they form a branched, root-like, complex 

 arrangement of the finest primitive fibrils. These are called 

 protoplasmic processes (I, y). By means of these processes, 

 rig. ^^6. adjoining cells are brought into communication with each 



Cell from the Gasserian ganglion, other, so that impulses can be conducted from one cell to 

 n, nuclei of the sheath; t, fibre another. Further, many of these fibrils approximate to each 

 dividing at a node of Ranvier. other and join together to form axis-cylinders of other nerve- 

 fibres. [V. Thanhoffer states that he has traced the axis-cylinder process to the nucleus and 

 nucleolus.] 



2. Bipolar cells are best developed in fishes, e.g., in the spinal ganglia of the skate, and in 

 the Gasserian ganglion of the pike. They appear to be nucleated, fusiform enlargements of 

 the axis-cylinder (fig. 368, on the right of I). The white substance often stops short on each 

 side of the enlargement, but sometimes the white substance and the sheath of Schwann pass 

 over the enlargement. 



3. Nerve-cells with connective-tissue capsules occur in the peripheral ganglia of man (fig. 



