OHAPTE-R XVII. 



THE SENSES. 



General Considerations. Through the medium of the nervous sys- 

 tem the mind obtains a knowledge of the existence hoth of the various 

 parts of the body, and of the external world. This knowledge is based 

 upon sensations resulting from the stimulation of certain centres in the 

 brain, by irritations conveyed to them by afferent nerves. Under normal 

 circumstances, the following structures are necessary for sensation: (a) 

 A peripheral organ for the reception of the impression ; (5) a nerve for 

 conducting it ; (<?) a nerve-centre for feeling or perceiving it. 



Classification of Sensations. Sensations may be conveniently classed 

 as (1) common and (2) special. 



(1.) Common Sensations. Under this head fall all those general 

 sensations which cannot be distinctly localized in any particular part of 

 the body, such as fatigue, discomfort, faintness, satiety, together with 

 hunger and thirst, in which, in addition to a general discomfort, there is 

 in many persons a distinct sensation referred to the stomach or fauces. 

 In this class must also be placed the various irritations of the mucous 

 membrane of the bronchi, which give rise to coughing, and also the 

 sensations derived from various viscera indicating the necessity of ex- 

 pelling their contents; e.g., the desire to defaecate, to urinate, and, in 

 the female, the sensations which precede the expulsion of the foatus. 

 We must also include such sensations as itching, creeping, tickling, 

 tingling, burning, aching, etc., some of which come under the head of 

 pain: they will be again referred to in describing the tactile sense. It 

 is impossible to draw a very clear line of demarcation, between many of 

 the common sensations above mentioned, and the sense of touch, which 

 forms the connecting link between the general and special sensations. 

 Touch is, indeed, usually classed with the special senses, and will be 

 considered in the same group with them; yet it differs from them in 

 being common to many nerves. Among common sensations some would 

 rank the muscular sense, which has been already alluded to. It is by 

 means of this sense that we become aware of the condition of the mus- 

 cles, and thus obtain the information necessary for their adjustment to 

 various purposes standing, walking, grasping, etc. This muscular 

 sensibility (to which we shall again refer) is shown in our power to esti- 

 42 657 



