OF SENSATION IN GENERAL. 385 



be the diaphragm and abdominal muscles, the conjoint actions of which would 

 be a peculiarity observed in no other instance. By Dr. M. Hall, on the other 

 hand, it is maintained that the act of vomiting is, like the expulsion of the 

 foetus, urine, faeces, &c., an expiratory effort, modified in its effects by the pe- 

 culiar condition of the sphincters. It bears, indeed, great resemblance to the 

 act of coughing ; differing chiefly in this, that in vomiting, the larynx is closed 

 during the whole operation, whilst it is only closed momentarily in coughing ; 

 and also, that in coughing, the cardiac orifice of the stomach is closed, whilst 

 in vomiting it is opened. In this view, the accuracy of which has been proved 

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

 corded 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 re- 

 markable change in the relative position of the viscera, the stomach lying in 

 the thorax, which cavity communicated with the abdomerf, 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 mus- 

 cles would have the same power of emptying the stomach, as they would 

 possess over the lungs. There can be little doubt, however, that the walls of 

 the stomach participate in the action; for even the oesophagus is thrown into 

 a. state of reversed peristaltic movement. 



CHAPTER VI. 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



1. Of Sensation in General. 



506. 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 defined 

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

 true, spoken of a sensation without consciousness ; but it seems very de- 

 sirable 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 Encephalon only, that the Mind possesses the 

 relation necessary for the production of such a change in it. Hence this 

 organ is spoken of as the Sensorium. For the reasons already given ( 435), 

 it seems probable that the ganglia of Special Sensation are rather the essential 

 instruments of this function, than the Cerebral Hemispheres. The afferent 

 nervous fibres, which connect the various parts of the body with the Senso- 

 rium, 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 ; 



* Dublin Hospital Reports, Vol. v. 

 33 



