654 SPECIAL SENSES. 



contact at the ends of the nerve, and there is no contact. The impression, 

 in order to be perceived at all, must be painful. These facts may be in a 

 measure applied to local impressions produced by extremes of heat and cold 

 or by chemical or electric stimulation of sensitive parts. 



The internal organs have as a rule no tactile sensibility, although they 

 may be sensitive ; and feeble impressions may not be appreciated, while 

 stronger impressions are painful. 



Titi Ration is the result of unusual, feeble impressions or of slight impres- 

 sions frequently repeated in the peripheral ends of certain sensory nerves. 

 These impressions are not precisely tactile nor are they painful. They pro- 

 duce peculiar sensations, and they frequently give rise to violent reflex move- 

 ments, by what is known as a summation of sensory stimulations. 



Muscular Sense (so called}. It is difficult to define exactly what is meant 

 by the term muscular sense, as it is used by some physiologists. In all proba- 

 bility, the sense which enables one to appreciate the resistance, immobility 

 and elasticity of substances that are grasped or stood upon or which are in 

 any way opposed to the exertion of muscular power, may be greatly modified 

 by education and habit. It is undoubtedly true, however, that general sensi- 

 bility regulates the action of muscles to a considerable extent. If, for exam- 

 ple, the lower extremities be paralyzed as regards sensation, the muscular 

 power remaining intact, frequently the person so aifected can not walk unless 

 he be able to see the ground. This difficulty occurs for the reason that the 

 limbs have lost the sense of contact. Many curious examples of this kind 

 are to be found in works upon diseases of the nervous system. One of the 

 most striking is a case communicated to Charles Bell by Dr. Ley. The 

 patient was affected with partial loss of sensibility upon one side of the body, 

 " without, however, any corresponding diminution of power in the muscles 

 of volition, so that she could hold her child in the arm of that side so long 

 as her attention was directed to it ; but if surrounding objects withdrew her 

 from the notice of the state of her arm, the flexors gradually relaxed, and the 

 child was in hazard of falling." This is like certain of the phenomena 

 observed in cases of locomotor ataxia. In this disorder there is disease of 

 the posterior white columns of the spinal cord, involving, sometimes, the 

 posterior roots of the spinal nerves, with more or less impairment of general 

 sensibility, the muscular power in some instances being intact. Patients 

 affected in this way frequently are unable to walk or stand without the aid 

 of the sight. One of the most characteristic phenomena is inability to stand 

 when blindfolded ; although, with the aid of the sight, the muscles can be 

 made by the will to act with considerable power. Habit and education enable 

 some persons to appreciate with great nicety slight differences in weight ; but 

 this is due chiefly to the sense of resistance to muscular effort and has little 

 dependence upon the sense of touch. 



In general those parts which are most sensitive to the impressions of 

 touch, as the fingers, enable one to appreciate differences in pressure and 

 weight with greatest accuracy. The sense of simple pressure, unaided by 

 the estimation of weight by muscular effort, generally is more acute upon 



