ON TIIE SENSES IN GENERAL. 513 



naturalist a greater power of distinguishing colors and outlines, 

 and which enables the physician to detect nice variations of quality 

 in the sounds of the heart or the respiratory murmur of the lungs. 

 The impressions of external objects, therefore, in order to produce 

 their complete effect, must first be received by a sensitive appa- 

 ratus, which is perfect in organization and functional activity; 

 and, secondly, these impressions must be subjected to the action of 

 an intelligent perception, by which their nature, source and rela- 

 tions may be fully appreciated. 



That part of the nervous system which we have hitherto 

 studied, viz., the cerebro-spinal system, consists of an apparatus of 

 nerves and ganglia, destined to bring the individual into relation 

 with the external world. By means of the special senses, he is 

 made cognizant of sights, sounds, taste, and odors, by which he 

 is attracted or repelled, and which guide him in the pursuit and 

 choice of food. By the general sensations of touch and the volun- 

 tary movements, he is enabled to alter at will his position and 

 location, and to adapt them to the varying conditions under which 

 he may be placed. The great passages of entrance into the body, 

 and of exit from it, are guarded by the same portion of the nerv- 

 ous system. The introduction of food into the mouth, and its 

 passage through the oesophagus to the stomach, are regulated by 

 the same nervous apparatus ; and even the passage of air through 

 the larynx, and its penetration into the lungs, are equally under 

 the guidance of sensitive and motor nerves belonging to the 

 cerebro-spinal system. 



It will be observed that the above functions relate altogether 

 either to external phenomena or to the simple introduction into the 

 body of food and air, which are destined to undergo nutritive 

 changes in the interior of the frame. 



If we examine, however, the deeper regions of the body, we find 

 located in them a series of internal phenomena, relating only to 

 the substances and materials which have already penetrated into 

 the frame, and which form or are forming a part of its structure. 

 These are the purely vegetative functions, as they are called ; or 

 those of growth, nutrition, secretion, excretion, and reproduction. 

 These functions, and the organs to which they belong, are not 

 under the direct influence of the cerebro-spinal nerves, but are 

 regulated by another portion of the nervous system, viz., the 

 " ganglionic system ;" or, as it is more commonly called, the " sys- 

 tem of the great sympathetic." 

 33 



