794 A MANUAL OF PHYSIOLOGY 



it would be an error to conclude that all do so in any animal, or that 

 all animals are in this respect alike. It is indeed extremely probable 

 that in different species of animals, and even in individuals of the 

 same species, there are considerable differences in the extent of the 

 sensory decussation in the cord, just as there are in the extent of the 

 motor decussation in the bulb. In some animals the greater part of 

 the sensory path may decussate in the cord ; in others the greater 

 part may decussate in the bulb, or higher up. The lack of 

 agreement in the experimental results may be due partly to this 

 cause. When it is further remembered how difficult it sometimes 

 is to interpret the aocount which a man gives of his sensations and 

 to recognise precisely the degree and nature of sensory defects 

 produced by disease in the human subject, it will not be thought 

 surprising that experiments on animals, from the time of Galen 

 onwards, should have yielded evidence which, although perhaps now 

 at length tending to a definite result, is still unfinished and in part 

 conflicting. 



If, leaving them out of account, not as valueless but as still 

 difficult of interpretation, we attempt to draw any general 

 conclusion from the clinical observations which, however im- 

 perfect, are in such questions our surest guide, it can only be 

 this, that in man some of the sensory impulses, and particularly 

 those connected with the cutaneous sensations of pain and tempera- 

 ture, decussate, in part at least, in the cord. But there is also 

 evidence that tactile afferent impulses, including those coming 

 from the muscles and related to the muscular sense, and, it may be, 

 some of the impulses associated with pain, decussate, not in the 

 cord, but in the bulb. 



The Paths for Different Kinds of Sensory Impressions. If this is 

 the state of our knowledge where the problem is merely to determine 

 the crossing-place of afferent impulses which are certainly known 

 to cross, it is only to be expected that we should be still more in 

 the dark as regards the routes by which different kinds of afferent 

 impulses thread their way through the maze of conducting paths in 

 the neural axis to reach their planes of decussation and gain the 

 ' sensory crossway ' in the internal capsule. Some authors have 

 indeed cut the Gordian knot by assuming that any kind of sensory 

 impression may travel up any afferent path. Direct stimulation of 

 a naked nerve-trunk, it has been argued in favour of this view, gives 

 rise to a sensation of pain ; stimulation of the skin in which the 

 end-organs of the nerve lie gives rise to a sensation of touch or a 

 sensation of temperature, according as the stimulus is a mild 

 mechanical or a thermal one, the contact of a feather or of a hot 

 test-tube. Why, it has been asked, should we imagine that the 

 difference in the result of stimulation depends on a difference in 

 the nerve-fibres excited, and not on a difference in the kind of 

 impulses set up in the same nerve-fibres ? This is a question which 

 we shall have again to discuss (p. 968). But apropos of our present 

 problem, we may say that there is very clear proof from the patho- 

 logical side that a limited lesion in the conducting paths of the 

 central nervous system may be associated with defect or total loss 

 of one kind of sensation, while all the other kinds remain intact. 



