224 OF SENSATION, AND THE ORGANS OF THE SENSES. 



has been proved by experiment, the diaphragm is quite inert. A curious case 

 has been recorded by Drs. Graves and Stokes,* in which vomiting took place 

 from the stomach of a man, who was found after death to be the subject of a 

 very remarkable change in the relative position of the viscera, the stomach 

 lying in the thorax, which cavity communicated with the abdomen by an 

 opening in the diaphragm, giving passage to the oesophagus and duodenum. 

 This case was regarded by its reporters as proving that vomiting might take 

 place by the action of the stomach alone ; but it can scarcely be held to justify 

 this conclusion ; since, the diaphragm being entirely passive, the abdominal 

 muscles would have the same power of emptying the stomach as they would 

 possess over the lungs. The conformity of the act of vomiting with that of 

 expiration, is further shown by the ejection of the contents of the oesophagus, 

 which will take place, when it is distended by the deglutition of food that can- 

 not pass into the stomach, on account of an obstruction at the cardia. 



CHAPTER IV. 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



I. Of Sensation in general. 



302. BY the term Sensation is rightly understood that change in the con- 

 dition of the mind, by which we become aware of an impression made upon 

 some part of the body ; or, in a briefer form of expression, it may be denned 

 to be the consciousness of an impression. Some physiologists have, it is true, 

 spoken of a sensation without consciousness ; but it seems very desirable thus 

 to limit the term ; since the word impression may be very well applied to 

 designate the change produced in the afferent nerves by an external cause 

 up to the point at which the mind becomes conscious of it. We have seen 

 reason to believe that the impressions communicated to the Spinal Cord may 

 there excite motor actions, without occasioning true Sensation ; and it would 

 seem to be with the Brain only, that the Mind possesses the relation neces- 

 sary for the production of such a 'change in it. Hence the Brain is spoken of 

 as the Sensorium. For the reasons already given ( 261), it seems probable 

 that the ganglia of Special Sensation share in this function with the Cerebral 

 Hemispheres. The afferent nervous fibres, which connect the various parts 

 of the body with the Brain, are termed sensory. This term has also been 

 applied to those which terminate in the Spinal Cord ; but as the impressions 

 which these convey do not produce sensations, it seems desirable to avoid 

 thus designating them ; and the term excitor, proposed by Dr. M. Hall, is 

 much preferable. Every afferent spinal nerve, therefore, is made up of sen- 

 sory and of excitor fibres ; and these may be distributed in very different pro- 

 portions to different parts. Of the excitor fibres, enough has been already 

 said. Those parts of the body which are endowed with sensory fibres, and 

 impressions on which, therefore, give rise to sensation, are ordinarily spoken 

 of as sensible ; and different parts are spoken of as sensible in different de- 



* Dublin Hospital Reports, vol. v. 



