Before a branch separates, it often happens that the parent trunk presents an enlargement for some distance above the point of visible separation. This is due to the fact that the fibres which compose the future branch begin to loosen their connection with the trunk for some way before they actually leave it ; and the con necting areolar membrane becomes conse quently looser _and more abundant. Hence the trunk of the nerve appears enlarged, with out any increase in the number of its nervous elements. This may be well seen in the auri cular nerve of the neck, as it winds upwards over the sterno-mastoid muscle.
Anastomosis of nerves.—In their bmnchings nerves subdivide, not only to pass immediately to their muscles or other distant parts, but also to connect themselves by certain of their filaments with other nerves, and to follotv the course of the latter, whether onward or re trograde, peripherad or centrad, instead of ad hering completely to that of the primary trunk. By these means, nervous filaments connected with very different parts of the brain and spinal cord become bound together in the same fasci culus, and a nerve is formed compounded of tube_s possessing very opposite functions. The anastomosis of nerves thus formed differs very obviously from the more correctly named anas tomosis of bloodvessels, for in the latter case the canals of the anastomosing vessels are made to communicate and their contents are mingled; but in the former the nerve filaments are simply placed in juxta-position. There is no fusion of the one into the other, no admixture of the pulpy contents of the nerve-tubes, which con tinue their course as perfectly insulated as if they were placed singly and had no connexion with others.
The simplest kind of anastomosis is that which occurs in the formation of almost every spinal nerve. The anterior and the posterior roots of these nerves, emerging frorn different parts of the spinal cord, and possessing, as is now proved, very different functions. are united after passing through the dura nutter, and bound together as one nerve; the com ponent tubules being so completely intermixed that the future ramifications of the nerve may enjoy the double function derived from the diverse endowment of the originally compo nent tubules.
And even in a nervous trunk thus formed there occurs a remarkable interchange of place between the component filaments, which are thereby made to decussate each other within the trunk of the nerve (fig. 332). Bichat says, " I amused myself one day in attentively following all the filarnents of the sciatic nerve some d istance .down the limb: those filaments which forme the exterior of the trunk above, I found, i greatest part, forming its centre below." Kronenberg states that in some nerves thes communications are so frequent that one canno follow a single fascicle for any distance; whilst in other nerves, as the external cutaneous nerve of the arm, he found some bundles which passed through a distance of upwards of six inches without uniting with neighbouring ones. This is an anatomical fact of no mean impor tance, as applicable to the explanation of many apparently anomalous symptoms in neuralgic and other nervous affections.
A second form of anastomosis may be best explained by referring to that with which all who have made the superficial dissection of the neck must be familiar,—namely, the anasto mosis of the descending branch of the ninth with the cervical plexus. Certain fibres, which pass 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 and apply themselves to a descending branch of the cervical plexus, fnrming in front of the caro tid artery and jugular vein an arch with the con cavity directed upwards, several nerves passing from the convexity to neighbouring muscles. A little careful dissection shows that some of the nervous filaments which are given off from the convexity are derived from the ninth nerve, and others from the descending branch of the cervical plexus ; whilst others seem to form a complete arch and to be equally connected with both nerves. If we trace them from the ninth nerve, we find them passing upwards and back wards into the descending branch of the cervical plexus, and so returning to the spinal cord. The nervous arch which is 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 appKirs to arise.