THE NERVES. 359 



They form a plexus about the vessels (which serve for their sup- 

 port), and proceed to the involuntary muscles (see n. sympathicus). 

 The more primitive fibres there are found in them, the more they 

 resemble the white nerves, e. g. the nerves of the heart. 



1. Course. Every primitive fibre passes uninterruptedly, and 

 without branches, from the central to the peripheral extremity, 

 generally outwards and downwards. The nerves mostly enter the 

 organs from within; or rather, they pass to their actually free 

 surface. 



2. Combinations take place in this way, only that fasciculi pass 

 over from one nervous trunk into another, the primitive fibres 

 always passing in an isolated condition, never coalescing with one 

 another. The different kinds of combination are as follows : 



a. Anastomosis, loops. A part of the fasciculus of a trunk is 

 detached, and passes away as a distinct branch (ramus). It 

 goes into the sheath of another trunk, and applies itself to 

 this, thus forming a simple anastomosis. When an inter- 

 change of fasciculi takes place between two trunks, a re- 

 ciprocal and decussating anastomosis arises. 



b. An interlacement, plexus, signifies the combination of anasto- 

 mosing branches into a network, the meshes of which are 

 empty, or are filled by other tissues. From each root of the 

 plexus each of the nerves passing off from it contains, at last, 

 separate fasciculi. Plexus gangliosus signifies a plexus, the 

 meshes of which are filled up with ganglion globules. 



c. Nervous ganglia, ganglion nervosum, is a smaller or larger 

 intumescence, formed by the accumulation of peripheral 

 ganglion globules. In addition to these, their sheaths and 

 vaginal processes, a larger or smaller number of primitive 

 fibres are found in it which pass through the ganglion, 

 singly, or forming a plexus, and entwining about the glo- 

 bules. The ganglion is surrounded by a strong neurilemma, 

 in which the capillary vessels ramify. 



612. 3. Origin and terminations. The roots of the nerves 

 are the immediate continuations of the primitive fibres of the brain 

 and spinal cord. The posterior roots contain finer, the anterior 

 coarser, fibres. United into fasciculi and trunks, they divide, as 

 before described, and, finally, spread out in the substance of the 

 organs in loops, so that the primitive fibre is reflected at its ex- 

 tremity and returns to the centrum, no proper termination to it 

 being present. 



The plexus, in which the terminations of the nerves resolve 



