CHAP, v.] THE SPINAL CORD. 601 



have a right to conclude that the afferent impulses which affect 

 blood-pressure take the same course as those which give rise to 

 sensations, and affect consciousness. And further, in all experi- 

 mental results on animals we must bear in mind the probability 

 that the functions of the spinal cord may vary in different animals, 

 possibly to a very considerable extent. 



In pathological cases we have the advantage of being able 

 clearly to define sensation and volition, but this is frequently more 

 than counterbalanced by the diffuse nature of the injury or disease, 

 and the want of exact anatomical verification. When these facts 

 are borne in mind, it will easily be understood that in no part of 

 physiology are the statements of investigators more conflicting and 

 unsatisfactory. 



One salient fact comes out in all observations, whether ex- 

 perimental or pathological, viz. that between the brain, where 

 volitional impulses are started, or where conscious sensations are 

 perfected, and the muscle which carries out the movement or the 

 sentient surface where the sensory impulses begin, there is a 

 complete crossing or decussation of all impulses whether sensory or 

 motor. When the right side of the brain is injured or diseased, 

 when for instance damage is done to the right corpus striatum and 

 optic thalamus, it is on the left side of the body, in the left limbs, 

 and in the left face, that the paralysis and loss of sensation appear ; 

 it is on the left side that sensory impulses fail to 'affect conscious- 

 ness, it is on the left side that the muscles can no longer be 

 reached by volitional impulses. Kesults other than these indicate 

 complications involving the other side of the brain. 



Further, all observers are agreed that as far as the spinal 

 nerves are concerned (and since we are now treating of the spinal 

 cord we may for the present leave out of consideration the cranial 

 nerves), the decussation is complete at about the level of the 

 upper part of the medulla oblongata and pons Varolii when the 

 paths are traced upwards ; that is to say, all the sensory impulses 

 coming from, and all the volitional impulses passing to, the left 

 side of the body, make their way along the right cms cerebri. 

 Nearly all observers again are agreed that the sensory impulses 

 cross over lower down in the spinal cord than do the volitional 

 impulses; but opinions differ as to the exact difference in the 

 paths of the two kinds of impulse. Experiments conducted by 

 some observers have seemed to shew that transverse division of the 

 lateral half of the cord in any part of its course below the medulla 

 oblongata is followed on the same side, below the injury, by loss of 

 voluntary movement, accompanied by no loss of sensation, but 

 even by increased sensitiveness or hypersesthesia, and on the 

 opposite side by loss of sensation without any affection of voluntary 

 movement. From these and other experiments these authors 

 conclude that sensory impulses entering into the cord at a posterior 

 root immediately cross to the other side of the cord and so ascend 



