NERVOUS SYSTEM. (NERVE.) 



594 



the separation which takes place in the fat cells 

 between the solid and the fluid elements of fat. 

 As the combination of the primitive muscular 

 fibres, in a common sheath, forms the muscle, 

 so the union of the primitive nervous fibres, in 

 a similar way, forms the nerve. And as the 

 primitive fibre of muscle passes undivided from 

 one point of the muscle to another, so the nerve 

 tube exhibits no subdivision in its course. 



Brandling of' nerves. The main trunk of a 

 nerve breaks up into its component bundles, as 

 it passes from centre to periphery, yielding up 

 branches to the various parts it is destined to 

 connect with the nervous centre. These branches 

 generally come off at acute angles, and soon 

 plunge into the muscles and other parts to which 

 they tend, dividing and subdividing as they pro- 

 ceed. Such is the most common mode of sub- 

 division, but there are many exceptions : some- 

 times a branch separates from the parent trunk at 

 an acute angle, and then turns to run in an op- 

 posite direction, forming an arch, from the con- 

 vexity of which several branches are given off. 

 Such a nerve is said to be recurrent; the in- 

 ferior laryngeal nerve takes this course. The 

 anastomotic arches between the emerging spinal 

 nerves, round the vertebral laminae, are also 

 exceptions to the separation at acute angles. 



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 branchings 

 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 follow 

 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 

 tubes possessing very opposite functions. The 

 aniixtomosis 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 from different 

 parts of the spinal cord, and possessing, as 

 is now proved, very different functions, are 

 united after passing through the dura mater, 

 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 (fg. 332). Bichat says, " I 

 amused myself one day in attentively following 

 all the filaments of the sciatic nerve some distance 



Fig. 332. 



Diagram to show the decussation of the primitive fibres 

 within the trunk of a nerve. (After Valentin.) 



down the limb : those filaments which formed 

 the exterior of the trunk above, I found, in 

 greatest part, forming its centre below."* 

 Kronenberg states that in some nerves these 

 communications are so frequent that one cannot 

 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 artery, 

 and apply themselves to a descending branch of 

 the cervical plexus, forming 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 



* Anat. Generate, t. i. p. 128, ed. 1801. 



