386 HEALTH AND DISEASE 



accessory, supplying some of the muscles of the iieck and shoulder. 

 12. The hypoglossal, supplying the muscles of the tongue. 



The Functions of the Nervous System.— Having thus acquired 

 some knowledge of the anatomical and microscopical characters of the 

 nervous system, we may proceed to consider the purposes it fulfils in 

 the body. Of the two constituents of the nervous system, cells and 

 fibres, the cells are regarded as the organs by means of which impres- 

 sions are perceived and registered, and impulses to motion or secretion 

 generated, whilst the fibres are mere conductors extending between the 

 cells and the particular tissues to which the nerve-fibres issuing from the 

 cells are distributed. Those nerves which conduct impressions from one 

 or other of the organs of sense — eye, ear, mouth, nose, skin, &c. — to the 

 spinal cord or brain are called sensory or afferent fibres. Those which 

 conduct impulses from the cord or brain to the muscles or glands are 

 named "efferent" or "motor" nerves. The rapidity with which the 

 conduction of impressions or impulses is effected is very considerable. 

 It is obvious that if the knowledge of the proximity of food or of danger, 

 communicated by the senses, which are the outposts of the nervous 

 system, is to prove of service, it is necessary that the information should 

 be both accurate and prompt. The accuracy is provided for by the 

 special attributes of the several senses. By daylight, the eye aftbrds 

 most of the information required, though in the majority of animals the 

 ears are constantly on the alert against the approach of an invidious 

 foe. At night the faculties of hearing and smell are those which are 

 specially exercised, and in many predatory animals their acuteness rises 

 to a height of which we can form but a faint idea. 



By whichever of the senses the impressions are conveyed to the cells 

 of the central nervous system, it is imjiortant that the muscular responses 

 should be effected with promptitude. 



At first sight it might appear impossible to acquire any definite 

 knowledge of the swiftness with which sensory impressions of objects 

 aflecting the animal, and the motor impulses by which it responds to 

 them, are propagated. The speed of thought is proverbial, yet by the 

 application of electrical currents, the rapidity of which may be regarded 

 as covering short distances and intervals with no appreciable loss of 

 time, conclusions have been arrived at showing that nervous changes, 

 currents or waves, travel at a much slower speed than the electric 

 current. 



It has been ascertained that nervous impulses, whether in an afferent 

 or in an efierent nerve, that is, whether sensory or motor, in animals 

 as different as a frog and a horse or man, travel at the rate of about 



