NERVOUS SYSTEM, SENSES, AND SENSE ORGANS i8i 



the first gill-cleft and another long one running forward to the 

 region of the palate. Finally, the tenth or vagus (X), another 

 mixed nerve, is a complicated one, which not only gives off 

 forked branches to the remaining gill-openings, but the main 

 stem passes along the alimentary canal and sends nerves to its 

 muscles and to those of the heart, whilst another stem, which 

 separates from the nerve soon after it leaves the brain, supplies 

 the whole of the sensory system of the lateral line. 



Microscopically, a nerve is not the simple structure that it 

 appears to the naked eye, but is made up of an enormous 

 number of very fine fibres lying together side by side like the 

 separate wires of a telephone cable. Each of these fibres may 

 be of considerable length and is about one-tenth of the thickness 

 of a human hair. Actually, they are nothing more than fine 

 processes drawn out from star-shaped nerve cells situated in 

 the brain or spinal cord, the tissue of these organs being made 

 up entirely of cells of this nature. Any comparison of the nerves 

 with the wires of a telephone system is inevitably inaccurate, for, 

 whereas the telephone wires carry messages in one direction 

 only, each nerve contains fibres of two kinds, one carrying 

 messages or nervous impulses outwards, the other inwards. 

 The first or motor fibres carry impulses to the various muscles, 

 causing them to contract; to the glands, causing them to secrete 

 their special products; or to the stomach and intestines, to 

 accelerate or retard the processes of digestion. The sensory 

 fibres, on the other hand, carry nervous impulses to the brain 

 or spinal cord from the sense organs, conveying warning of 

 cold, hunger, pain, fear, and the like. As soon as these messages, 

 which generally follow some change in the conditions of the 

 outside world, are received, motor impulses are promptly sent 

 back along the nerves, and by an appropriate contraction or 

 expansion of certain muscles, matters are quickly adjusted. 

 The manner in which the various actions of the body are co- 

 ordinated by the central nervous system will be dealt with in 

 due course, and for the present it must suffice to point out 

 that every muscle-fibre is supplied by its own nerve-fibre, one 

 end of which forms a nerve cell in the brain or spinal cord 

 and the other terminates in a cluster of fine branches spread 

 out over the surface of the muscle-fibre. Every muscle is in 

 turn made up of a large number of separate fibres, and is served 

 by its own nerve, which controls its every action. When it is 

 considered that to perform the simplest movement, the waving 

 of a fin or the opening of the mouth, the co-ordinated action of 



