240 DISTRIBUTION OF NERVES TO STRIPED MUSCLE. 



nerve-trunks and nerve-fibres is one -wliicli is very 

 general, and was known even to the older anatomists. 

 PI. IX, fig. 2, p. 245. It can be demonstrated in many- 

 cases even by rough dissection. Plexuses exist not 

 only in the case of nerves distributed to muscle, but, 

 as far as is knowT^i, to every other tissue which re- 

 ceives a supply of nerves. Many of these networks 

 are very beautiful ; and the arrangement is illustrated 

 in many of my figures, particularly in those repre- 

 senting- the bundles of dark-bordered nerve-fibres dis- 

 tributed respectively to the diaphragm of the white 

 mouse, Plate VII, tig. 1, the mylohyoid of the gTeen 

 tree-frog, Plate VIII, fig. 1, and the eyelid of the 

 same animal. The fibres constituting the bundles 

 never run perfectly parallel, nor can a separate fibre 

 usually be followed for any great distance. This arises 

 from the fact that the fibres frequently cross one an- 

 other, and are seen to pursue a spiral course in many 

 instances. At an early period of development one 

 fibre may be seen coiled spirally round the other, as 

 is well shown in one of my drawings, fig. 2, Plate 

 VIII. See also my paper " On the Structure of the 

 so-called Apolar, Unipolar, and Bipolar Nerve-cells," 

 Phil. Trans., 1863. The rule seems to be universal 

 that fibres on one side of a nerve-trunk cross over 

 and pursue their course on the opposite side. Those 

 on the lower part of a trunk soon pass to the upper 

 part, and vice versa. Instead of a nerve passing to its 

 distribution by the shortest route, it invariably seems 

 to pursue a very circuitous coiirse. Nor is the cross- 

 ing of the nerve-fibres in the optic commissure 

 peculiar to this part of the nervous system, but a 

 similar arrangement is to be met with in all nerves. 

 At the point where two trunks seem to meet and 

 cross one another, it is easy to demonstrate, 1. Fibres 

 pursuing a direct course. 2. Fibres crossing from 

 one side to the other, o. Central commissural fibres. 

 4. Pei'ipheral commissural fibres. 



