424 THE NERVOUS SYSTEM. 



head briskly in consequence of a fly having alighted on the wound. 

 The discharge of a pistol behind his back will often cause him to open 

 his eyes and turn his head, as if in sign of having heard the report ; 

 but he immediately becomes quiet again and pays it no further atten- 

 tion. Yulpian found that in a pigeon, after the animal had been 

 roused by pinching the foot, the sudden approach of a hand toward 

 the eye caused a winking movement with partial turning of the head. 

 Sometimes such a pigeon will fix his eye on a particular object for 

 several seconds together ; and Longet found that on moving a lighted 

 candle before the bird in a dark place, its head would often follow the 

 movements, showing that the retina was still sensitive to light. 



But it is doubtful whether such movements indicate a real perception 

 on the part of the animal, or whether they are simply automatic reac- 

 tions of the nervous system, like the contraction and dilatation of the 

 pupil in a person who is unconscious. It is certain that, if impressions 

 are perceived by the pigeon after removal of the hemispheres, they are 

 immediately forgotten ; and furthermore that they do not excite any 

 corresponding series of ideas. The report of a pistol causes no sign 

 of alarm, and is not followed by any attempt at escape ; for the sound, 

 even if perceived by the animal, does not suggest any idea of danger or 

 injury. External phenomena, and their impressions on the nervous 

 system, are without significance for the animal ; and be is consequently 

 no longer capable of originating intelligent volitional acts. 



III. In man, the general result of injury or disease of the hemi- 

 spheres is a disturbance of the intellectual faculties. Among the earliest 

 and most constant of these phenomena is an impairment of memory. 

 The patient forgets the names of particular objects or persons ; or he 

 is unable to calculate numbers with his usual facility. His mental 

 derangement is often shown in the undue estimate which he forms of 

 passing events. He will show an exaggerated degree of solicitude 

 about a trivial occurrence, while he pays no attention to matters of real 

 importance. As the difficulty increases, he becomes careless of direc- 

 tions and advice, and must be managed like a child or an imbecile. 

 Finally, when the injury to the hemispheres is excessive, the senses 

 may still remain impressible, while the patient is completely deprived 

 of intelligence. ^ The frequency of these results in lesions of the hemi- 

 spheres, without loss of sensibility or motion, shows the close connec- 

 tion between the mental powers and the nervous action of this portion 

 of the brain. 



The same connection is seen in congenital idiocy with imperfect 

 development of the brain. In many cases the immediate condition 

 upon which idiocy depends is the small size of the brain as a whole, 

 and particularly that of the cerebral hemispheres. The general and 

 special senses, and the activity of the nervous system at large, are 

 sometimes fully developed in idiots, while the intelligence remains at 

 so low a grade, that no improvement in the mental operations is possi- 

 ble, and instruction is consequently without effect. 



