192 THE HUMAN BODY. 



they are only ordinary sensory or reflex fibres, and that the 

 inhibition is due only to the interference of two impulses 

 reaching a central organ at the same time and impeding or 

 hindering the full production of the normal result of either. 



In efferent nerve-fibres physiologists also distinguish sev- 

 eral groups. 1. Motor fibres, which are distributed to the 

 muscles and govern their contractions. 2. Vaso-motor fibres. 

 These are not logically separable from other motor fibres; 

 but they are distributed to the muscles of the blood-vessels, 

 and by governing the blood-supply of various parts, indirectly 

 produce such secondary results as entirely overshadow their 

 primary effect as merely producing muscular contractions. 

 3. Secretory fibres. These are distributed to the cells of the 

 Body which form various liquids used in it, as the saliva and 

 the gastric juice, and arouse them to activity. The salivary 

 glands, for instance, may be made to form saliva by stimulat- 

 ing nerves going to them, and the same is true of the cells 

 which form the sweat poured out upon the surface of the 

 Body. 4. Trophio nerve-fibres. Under this head are included 

 nerve-fibres which hate-^been supposed to govern the nutri- 

 tion of the various tissues, and so to control their healthy 

 life. It has been doubted if any such nerve-fibres exist as a 

 distinct class, and no doubt many of the facts which have been 

 cited to prove their existence are otherwise explicable. For 

 instance, shingles is a disease characterized by an eruption on 

 the skin along the line of certain nerves, oftenest those which 

 run between the ribs; but it may be dependent upon disease 

 of the vaso-motor nerves which control the blood-supply of 

 the part. In other cases diseases ascribed to injury of trophic 

 nerves have been shown to be due to injury of the sensory 

 nerves of the part, which having lost its feeling, is exposed to 

 injuries from which it would otherwise have been protected. 

 There are, however, cases which seem to indicate a direct niv 

 tritive influence of the nervous system on the tissues; as for 

 example the acute bedsores seen in some diseased states of the 

 spinal cord and leading to extensive destruction of the skin in 

 a very few hours; and there is direct experimental evidence 

 to show that stimulation of the branches of the pneumogastric 

 nerve going to the heart tends to restore that organ when ex- 

 hausted, Vhile stimulation of the sympathetic branches has a 

 precisely opposite effect (see Chapter XVIII). There is also 

 no doubt that each nerve fibre depends for the maintenance 



