710 THE BRAIN. 



and those which have remained on the same side. For, as we have already 

 said, we have evidence, in man at least and some other animals, that afferent 

 impulses cross completely over somewhere or other on their path before 

 they are developed into full sensations ; and we have also evidence, though 

 less strong, that they cross not long after their entrance into the cord. But, 

 if we suppose this to be the case in the rabbit also, it follows that in the 

 experiment in question the impulses which were blocked on their passage 

 along the lateral column of the same side, whatever the way by which they 

 reached that lateral column, were pursuing a path which would eventually 

 have led them to the other side of the cord. Hence, the section of the 

 lateral column, in breaking their path, broke not a continuous path keeping 

 to the lateral column up the length of the cord, but a path which soon left 

 the lateral column to pass elsewhere. The experiment, therefore, as far as 

 the impulses passing up the same side are concerned, does not prove that 

 they pursue a continuous path along the lateral column ; and if so, what be- 

 comes of the validity of the experiment as regards the impulses crossing over 

 from the other side? for the experiment in itself makes no distinction between 

 the two. 



We may add, however, that though the point has not been specially 

 investigated, it is possible that in the rabbit, in whose hind limbs bilateral 

 movements are so predominant, there is associated with the movements a 

 bilateral arrangement for sensations, and that those impulses which remain 

 along the same side of the cord as the nerve in which they originate, are 

 carried up to the brain without any crossing at all. 



594. The results of this vasomotor experiment then, though they are 

 frequently quoted, do not when closely considered afford adequate proof that 

 afferent impulses pursue a continuous path along the lateral columns of the 

 cord, and, moreover, the facts brought to light by the experiment show but 

 little accord with the anatomical programme. We have dwelt on it so long 

 because it is more or less illustrative of the many difficulties attending the 

 interpretation of experiments of this kind ; and it is in this respect all the 

 more valuable because the actual experimental results are sharp and clear. 

 We may pass over more rapidly the numerous experiments on the lower 

 mammals, such as rabbits and dogs, in which other indications of sensation 

 have been made use of, chiefly those which are the signs of painful sensa- 

 tions; these have been carried out in various regions of the cord, but chiefly 

 in the thoracic region, and in them a like uncertainty of interpretation is 

 further increased by the want of exactness and agreement in the results. 



If we content ourselves with making no distinction between the different 

 kinds of afferent impulses, and in the case of these animals it would hardly 

 be profitable to attempt to make a distinction, we may say that the several 

 experiments so far agree that they point to the lateral columns as being the 

 chief paths of afferent, sensory impulses, or to speak more exactly, to the 

 passage of these impulses being especially blocked by section of the lateral 

 columns. Some observers find that in the dog and other lower mammals a 

 section of the lateral column on one side, or at least a hemisection of the 

 cord, produces " loss of sensation " on the opposite side greater than on the 

 same side, or confined to the opposite side and even accompanied by an 

 exaltation of sensation, a hypersesthesia, on the same side. Other observers 

 again, and these certainly competent observers, find that, in the dog, section 

 of one side affects sensations on both sides, and, indeed, chiefly on the same 

 side. We may perhaps once more repeat the warning of the difficulty attend- 

 ing the quantitative and qualitative determination of sensations in such 

 an animal as the dog ; and may remark that in all these cases of unilateral 

 section the increased blood-supply due to failure of the normal vaso-cou- 



