220 INNERVATION. [CHAP. vm. 



nerve. There is a similar nervous arch formed between the second 

 or third cervical nerve and the accessory nerve. Certain fibres, 

 when traced from the former, appear to pass back to the centre 

 in the sheath of the latter. This anastomosis Volkmann found 

 in the human subject, and in several of the lower animals.* 



According to Gerber, similar loops are found in the sheaths of 

 spinal nerves. Certain fibres emerge from and return to the nervous 

 centre, forming a loop with the convexity directed towards the peri- 

 phery, without connecting themselves with any peripheral organ 

 or texture, or going beyond the nerve-sheath. To these loops this 

 anatomist has given the fanciful title, nervi nervorum. 



Plexuses. When several neighbouring nerves freely interchange 

 their fibres, a complicated form of anastomosis is produced, which 

 is called a plexus. Four or five nerves, for example, proceed from 

 the spinal cord, for a certain distance without any communication 

 with each other. A division of each then takes place, and from 

 the conjunction of their neighbouring branches new nerves result, 

 which again subdivide and interchange fibres; and by the free 

 communication which is thus established, a network is sometimes 

 formed, (as in the cervical plexus) , in the meshes of which areolar 

 tissue, and sometimes fat, are deposited. Finally, certain nerves 

 emerge from the plexus thus formed, which are composed of fibres 

 derived from several of the original trunks. Examples of this kind 

 of anastomosis are found in connexion with the anterior branches 

 of the spinal nerves in the neck, the axilla, the loins, and the sacral 

 region; and there are also plexuses formed in the course of the 

 fifth nerve, the portio dura of the seventh, the glosso-pharyngeal, 

 and the par vagum. 



The fibres, which pass through a plexus, notwithstanding the ap- 

 parent intricacy of their communication, preserve their individuality. 

 This may be proved by irritating a single nerve before it has broken 

 up in the plexus. Such irritation will produce contraction of cer- 

 tain muscles only ; of those, namely, to which the fibres of that 

 nerve are distributed. It is probable that, owing to the frequent 

 change of place which the fibres undergo within the plexus, they 

 are brought into communication with a greater number of muscles 

 than if no such subdivision had taken place. Kronenberg's expe- 

 riments shewed that the irritation of certain nerves of the plexus 

 before their subdivision caused the contraction of those muscles only 

 which received filaments from them.t 



* Miiller's Archiv., 1840. 



t Plexuum nervorum structura et virtutes. Berol. 1830. 



