THE NERVOUS PLEXUSES. 255 



gether with the roots of the lumbar and sacral nerves descending from 

 it, produce the appearance named the cauda equina, or horse's tail, 

 Fig. 60, e. 



On comparing the preceding description of the cranial with the 

 spinal nerves, it will be noticed that all the spinal nerves arise by two 

 roots, viz., a posterior ganglion-bearing root, and an anterior root 

 having no ganglion, and joining the other root beyond its ganglion. 

 Of the cranial nerves, however, the fifth nerve alone so far resembles 

 a spinal nerve as to have a double root, one ganglionated and the 

 other not; whilst the glosso-pharyngeal and pneumogastric have gan- 

 glia upon their trunks, and the remainder arise by single roots un- 

 provided with ganglia. We shall hereafter return to the subject of the 

 homology of the cranial with the spinal nerves. 



Nervous plexuses. In pursuing their course to the various tissues 

 which they supply, the branches of the nerves, both cranial and spinal, 

 always continue to divide and subdivide into smaller and smaller twigs, 

 until they arrive at minute filaments, or even at single fibres, which 

 terminate in various ways, in the tissues to which they are distributed 

 (p. 53). At some parts of their course, certain branches of the nerves 

 reunite again, so as to form angular networks, or meshes, called plex- 

 uses. Examples of these plexuses, are met with in certain junctions, 

 or anastomoses, of the fifth and facial nerves on the face, and in the 

 union of the pharyngeal branches of the glosso-pharyngeal and pneumo- 

 gastric nerves. Smaller meshes of anastomosis, or junctions, occur in 

 the branches of the same nerve, as in those of the olfactory nerves 

 beneath the nasal mucous membrane, and in the still more microscopic 

 interweaving of the fibres of the optic nerve in the retina, and of the 

 auditory nerve in certain parts of the internal ear. But very large 

 and remarkable plexuses are formed by the anterior branches of the 

 spinal nerves. Thus, the cervical plexuses are formed between the 

 first four cervical nerves, at each side of the neck; the so called bra- 

 chial or axillary plexuses, Fig. 60, ax, are composed of branches of 

 the four lower cervical and first dorsal nerve, at the root of the neck, 

 and give off the large nerves of the upper limb, Fig. 62 ; the lumbar 

 plexuses, Fig. 60, I, are formed by the four upper lumbar nerves; and, 

 finally, the great sacral plexuses, s, are formed by the last two lumbar 

 and four upper sacral nerves, from which the very large nerves of the 

 lower limb proceed, amongst them the great sciatic nerve, the largest 

 nerve in the body. In these plexuses, there is an interchange not 

 only of the funiculi or bundles of nerve fibres, but necessarily of the 

 nerve fibres themselves, belonging to various cranial or spinal nerves. 

 The nerve fibres may pass, from one nerve entering the plexus, into 

 all the nerves given off from it, and this in various degrees of inter- 

 mixture; these fibres themselves, however, never divide and unite, but 

 retain their continuity from the brain, or cord, to the localities of 

 their ultimate distribution. The effect of these plexuses is, that any 

 given nerve beyond the plexus, contains, or is composed of, nerve 

 fibres springing from a considerable length of the spinal cord, and 

 thus their purpose seem to be first, to establish a connection between 

 any one point of local distribution and a large extent of the nervous 



