280 DETAILED SYNTHESIS. 



also, commencing with those of the neck, give out nerves 

 that lead from each side downward and toward the cen- 

 tre till, meeting, or nearly meeting, they form inextrica- 

 ble plexuses, associated with ganglia, called the central 

 ganglia, and having specific names, cardiac, solar, &c. 



683. FROM THE CENTRAL GANGLIA and plexuses, 

 nerves extend in great numbers, especially following the 

 course of the large arteries to the organs of the trunk, 

 and, as it is thought, extending with the arteries into all 

 parts of the body. 



684. It CAN NOW READILY BE COMPREHENDED that 



with so many nervous centres, associated by millions of 

 fibres with all parts of the body, all the combinations 

 and sequences of action that can be desirable, are pro- 

 duceable; that action without and with sensation can 

 be provided for ; and that the body isj to a degree, a self- 

 regulating and also superintended machine, so arranged 

 that a cause acting on one part may, through a common 

 centre, affect a very distant part, which again may exert 

 influences upon half a dozen parts, all of which may 

 concentrate their influences on the first part, or still 

 other intervening steps may be necessary. 



685. Illus. A PARTICLE OP DUST is INHALED, and proves annoying 

 by influence through a sensatory nerve ; but that is not sufficient for its 

 removal. Another centre is at the same time acted upon, and influences 

 from it gush down upon the muscles of inspiration, contracting them, 

 succeeded by influences closing the mouth and windpipe, which acts ex- 

 ert their influence, and the expiratory muscles,contract, followed by the 

 sudden relaxation of those closing the windpipe, and continued contrac- 

 tion of those closing the mouth, and the air is forced rapidly through 

 the nose, sweeping away the obnoxious particles. At the same time 

 other influences or branches of the same influence are exerted upon the 

 gland that supplies the tear-fluid, which it pours copiously over the eye. 

 The fluid, coursing rapidly to the inner angle, finds its way down the ducts 

 into the nose and helps wash away the obnoxious matter. Thus horse- 

 radish, mustard, and other volatile, irritating substances^ act on the lining 

 of the back part of the nose, and produce a corresponding series of effects. 



Describe Fig. 192. Fig. 193. 688. How do nerves extend? 684 What -f 

 685. Describe effect of. 



