CHAPTER XX. 



THE SENSES. 



General Considerations. Through the medium of the Nervous sys- 

 tem the mind obtains a knowledge of the existence both 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 (sensory) nerves. 

 Under normal circumstances, the following structures are necessary for 

 sensation: (a) A peripheral organ for the reception of the impression; 

 (b) a nerve for conducting it; (c) 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 sen- 

 sations derived from various viscera indicating the necessity of expelling 

 their contents; e. g., the desire to defsecate, 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 sense of Touch. It is impos- 

 sible to draw a very clear line of demarcation between many of the com- 

 mon 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; e. g., all the sensory spinal nerves, the vagus, glosso- 

 pharyngeal, and fifth cerebral nerves, and in its impressions being com- 

 municable through many organs. Among common sensations must also 

 be ranked the muscular sense, which has been already alluded to. It is 

 by means of this sense that we become aware of the condition of con- 



