634 NERVOUS SYSTEM. 



" 9. External part of the anterior pyramids (Magendie) ; 



" 10. Portion of the medulla from which the facial nerve arises (Brown- 

 Sequard) ; 



" 11. Optic nerves; 



"12. Semicircular canals (Flourens) ; auditory nerve (Brown-Sequard)." 



To the parts above enumerated, Vulpian added the upper part of the 

 cervical portion of the spinal cord. 



The movements which follow unilateral injury of the parts mentioned 

 above are of two kinds ; viz., rolling of -the entire body on its longitudinal 

 axis, and turning, always in one direction, in a small circle, called by the 

 French the movement of manege. A capital point to determine in these 

 phenomena is whether the movements be due to paralysis or enfeeblement 

 of certain muscles upon one side of the body, to a direct or reflex irritation 

 of the parts of the nervous system involved or to both of these causes com- 

 bined. The experiments of Brown-Sequard and others show that the move- 

 ments may be due to irritation alone, for they occur when parts of the en- 

 cephalon and the upper portions of the cord are simply pricked, without 

 section of fibres. When there is extensive division of fibres, it is probable 

 that the effects of the enfeeblement of certain muscles are added to the phe- 

 nomena produced by simple irritation. 'The most satisfactory explanation 

 of these movements is the one proposed by Brown-Sequard, who attributed 

 them to a more or less convulsive action of muscles on one side of the body, 

 produced by irritation of the nerve-centres. He regarded the rolling as 

 simply an exaggeration of the turning movements, and places both in the 

 same category. 



It is not necessary to enter into an extended discussion of the above ex- 

 periments. In some of them, the movements have been observed toward 

 the side operated upon, and in others, toward the sound side. These differ- 

 ences probably depend upon the fact that in certain experiments, the fibres 

 are involved before their decussation, and in others, after they have crossed 

 in the median line. In some instances, the movements may be due to a reflex 

 action, from stimulation of afferent fibres, and in others, the action of the 

 irritation may be direct. Judging from the fact that most of the encephalic 

 commissural fibres are apparently insensible and inexcitable under direct 

 stimulation, it is probable that the action generally is reflex. 



