CHAP. IX.] NEBVOUS SYSTE3I AND OEGANS OF SENSE. 311 



the fi'og, respiration will continue if the medulla be left intact, even 

 thougli the brain and the spinal cord have both been removed, while 

 destruction of the medulla oblongata immediately arrests it. Injury 

 to a limited region of the floor of the fourth ventricle causes sudden 

 cessation of respiration and instant death in warm-blooded animals. 

 The fact that so many cranial nerves have their deep origins in this part 

 of the cerebro-spinal axis, sufficiently explains the extensive character 

 of its excito-motor and sensori-motor activities. Indeed this part 

 may be supposed to serve as the organ of the common sense, i.e., of 

 that sense by which the several senses come into relation one with 

 another and are felt as diverse modifications of one sentient being. 



§ 34. The FUNCTioxs of the pons varolii are, at least, in part 

 similar to the conducting functions of the medulla — the central grey 

 and white portions of both being continuous. Animals deprived of 

 both cerebrum and cerebellum, but with the medulla and pons 

 intact, remain motionless if undisturbed, but if disturbed move 

 themselves into postures of stable equihbrium, which they seem 

 incapable of doing if the pons be removed, although the medulla 

 be left. 



If the fibres passing from the pons to the cerebellum on one side 

 be cut through, the injured animal's body begins to rotate towards 

 the injured side, the eyeball of the injured side moving downwards, 

 while the eye of the opposite side rolls convulsively. 



The FUNCTioxs of the corpora quadrigemixa are closely con- 

 nected with the sense of sight, since not only their destruction causes 

 blindness and immobility in the iris, but they often waste after 

 atrophy of the eyes. The destruction of one side of this structure 

 causes blindness in the opjjosite eye. 



The OPTIC THALAMi are also thought to be concerned in sight, yet 

 this sense may persist after their destruction. They are by many 

 supposed to form the anterior termination of the sensory tract of the 

 cerebro-spinal axis and to be the organ of the common sense, which 

 sense, however, may pei-haps be more fitly attributed to the medulla 

 oblongata. There is evidence that they have to do with the power 

 of supporting the body on the limbs, as this power may remain after 

 the destruction of the corpora striata, but ceases after the removal 

 of the optic thalami. 



The CORPORA STRIATA are similarly deemed by many to form the 

 anterior termination of the motor tract of the cerebro-spinal axis 

 and to convey downwards from the cerebrum impulses resulting from 

 the grouping of images attributed to the cerebral hemispheres. 



The CEREBELLUM has had a variety of functions assigned to it, 

 but its real ofiice is stiU unascertained. It has been supposed to 

 co-ordinate the muscular movements, or to specially direct the 

 muscles of the eyes, or to be the organ of space relations generally, 

 or finally, to be the seat of the sexual emotion. 



On the whole it appears probable that the cerebellum ministers to 

 complex and co-ordinated muscular movements. Such a conclusion 

 seems to result from pothological facts and vivisections : animals 



