750 SPECIAL SENSES. 



pressions only ; but it is evident that the filaments of peripheral distribution of these 

 nerves are capable of receiving a variety of impressions, by which we determine, to a cer- 

 tain extent, the form, size, character of surface, density, and temperature of objects. We 

 also have a general appreciation of heat and cold ; a sense of resistance, which gives an 

 idea of weight ; and, finally, there are nerves of peculiar properties, terminating in organs 

 calculated to receive the impressions of smell, taste, sight, and hearing. 



The senses of olfaction, gustation, vision, and audition, belong to peculiar organs, pro- 

 vided with nerves of special properties, which are usually not endowed with general sen- 

 sibility. These nerves have been omitted in our general study of the nervous system ; 

 and the accessory organs to which they are distributed are so important and intricate in 

 their structure as to demand extended description. 



The senses of touch, titillation, temperature, and pain are all conveyed to the nerve- 

 centres by what we have described as ordinary sensory nerves ; the touch being perfected 

 in certain parts by peculiar arrangements of the terminal nerve-fibres. Although it be 

 possible that each one of these impressions may be transmitted by special and distinct 

 fibres, this has not yet approached a positive demonstration. The so-called muscular 

 sense, by which we appreciate weight, resistance, etc., undoubtedly depends, to a great 

 extent if not entirely, upon the muscular nerves. 



Muscular Sense (so called). 



It is difficult to define exactly what is meant by the term muscular sense, as it is used 

 by many physiologists. In all probability, the sense which enables us to appreciate the 

 resistance, immobility, and elasticity of substances that are grasped, on which we tread, 

 or which, by their weight, are opposed to the exertion of muscular power, is immensely 

 modified by education and habit. Still, it is undoubtedly true that the general sensibility 

 regulates the action of muscles to a very great extent. If, for example, the lower extremi- 

 ties be paralyzed as regards sensation, the muscular power remaining intact, the person 

 affected frequently cannot walk, unless he be able to see the ground. This difficulty 

 occurs for the simple reason that the limbs have lost the sense of contact with the ground, 

 which is nothing more nor less than loss of general sensibility. 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 Sir Charles Bell by Dr. Ley. The patient was 

 afflicted 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 sur- 

 rounding objects withdrew her from the notice of the state of her arm, the flexors grad- 

 ually relaxed, and the child was in hazard of falling." This is like certain of the phe- 

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

 posterior 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 are frequently 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 great power. 



Without entering into a full discussion of the various arguments used for and against 

 the existence of a special " muscular sense," it is sufficient to state that, in those cases 

 in which general sensibility is lost or seriously impaired, the brain has no exact apprecia- 

 tion of the action of the muscles, except as regards the sense of fatigue. This question 

 is of great importance in connection with the pathology of the nervous system ; and it 

 seems that the weight of evidence is decidedly in favor of the view that there is no dis- 

 tinct perception of muscular action, aside from general sensibility, that can properly be 

 called a muscular sense. 



