MUSCLE 269 



child is required to focus his eyes upon a printed page without 

 the aid of spectacles, not headache merely, but actual disease 

 of the brain, may be the result. The ciliary muscle within the 

 eyeball, which effects accommodation of the eye for near objects, 

 is unduly strained. Even the use of our modern type, with 

 its vertical height greater than its breadth, which has taken 

 the place of square Roman letters, is probably related to the 

 development of astigmatism of the lens, and thus indirectly 

 a cause of headache. It is asserted on high authority that 

 vertical astigmatism, the commonest form, is not present in 

 the eyes of children before they learn to read. Headache is 

 an exaggeration of the feeling of fatigue. It may be inter- 

 preted as the brain's expression of unwillingness to be made to 

 work ; a protest always to be listened to, notwithstanding that 

 it does not necessarily follow that unwillingness to work is the 

 result of overwork. Constipation, irritation of the sensory 

 nerves of the stomach, overdosing of the brain with alcohol, 

 and many other causes, may, through the vaso-motor system, 

 set up the conditions which normally result from activity un- 

 duly prolonged. The fact that a central disturbance, headache, 

 results from undue muscular work calls our attention to the 

 double nature of the mechanism concerned in movement. 

 Muscles are set in motion through the intervention of the 

 nervous system. After they have worked to an unusual 

 extent the nerve-centres connected with them grow tired. 

 This, at least, is a legitimate inference from the fact that 

 headache occurs when certain muscles of the eyeball have been 

 subjected to an improper strain. But it must be remembered 

 that the muscles of the eyeball never tire. They do not, like 

 other voluntary muscles, give notice that they are in need of 

 rest. It is not so clear that the central mechanism is in any 

 way involved in the fatigue which is produced by excessive 

 use of arms or legs. The muscles of the limbs (and the central 

 nervous system) are protected by the sensations which originate 

 in muscles when they are overworked. The fact that a 

 weary man can, if a great emergency demands activity, use his 

 muscles with as much vigour as if he were fresh from bed, has 

 been cited as an argument in favour of the view that fatigue 

 is of central origin ; but it is an argument which works both 

 ways. A strong emotion causes a fervent response from the 



