CHAP, viii.] ANASTOMOSES OF NERVES. 219 



According to Kronenberg, this kind of interchange is more frequent 

 in some nerves than in others ; and it is stated by this author that 

 in the external cutaneous nerve of the arm he found some bundles, 

 which passed through a distance of six inches without uniting with 

 neighbouring ones. 



By another form of anastomosis nervous loops or arches are formed, 

 the convexities of which are directed towards the periphery, and give 

 off filaments to the neighbouring parts. The well-known anastomosis 

 between the ninth or hypoglossal nerve, and the cervical plexus, in 

 front of the carotid artery, may be quoted as a good example. 

 Certain fibres, which come from the medulla oblongata as part of 

 the ninth nerve, leave that nerve as it crosses over the carotid 

 artery, pass down in front of the artery, and apply themselves to a 

 descending branch of the cervical plexus, forming in front of the 

 carotid artery and jugular vein an arch with the concavity directed 

 upwards, several nerves passing from the convexity to neighbouring 

 muscles. Some of the filaments which are given off from this arch, 

 are derived from the ninth nerve, and others from the cervical 

 plexus; whilst others seem to form a complete arch, and to be 

 equally connected with both nerves ; and, if we trace these latter 

 fibres from the ninth nerve, we find them passing upwards and 

 backwards into the descending branch of the cervical plexus, and 

 so returning to the spinal cord. The nervous loop, thus formed, 

 must evidently establish a communication between the cervical 

 region of the spinal cord and that portion of the medulla oblongata 

 whence the ninth nerve appears to derive its origin. 



Similar nervous loops, leaving the nervous centre as a constituent 

 of one nerve, and returning to it at some distance in company with 

 a different nerve, are found in various parts of the nervous system. 

 The commissural fibres of the optic tracts may be quoted as an 

 example. These fibres leave the centre by one tract, and return to 

 it by the other. It is probably owing to an anastomosis of this 

 kind, between the posterior and anterior roots of the spinal nerves, 

 that the latter enjoy a slight degree of sensibility. Other instances 

 of a similar kind have been described by Volkmann. In the calf 

 he found an anastomosis between the fourth pair of nerves and the 

 first branch of the fifth pair, forming an arch, from the convexity 

 of which several branches passed off in the peripheral direction. 

 By far the greater part of these, on microscopic examination, ap- 

 pt'invd to receive their fibres from the fourth; while those fibres 

 of the fifth, which contributed to the formation of the arch, passed 

 centripetally to the brain, bound up in the sheath of the fourth 



