372 PHYSIOLOGY 



All these tracts are mixed, i.e. contain both ascending and de- 

 scending fibres. As a rule, the longer the course of a fibre the more 

 peripherally does it lie in the cord. The shortest of the fibres may 

 only unite segment to segment, while the longest fibres may run 

 through the greater part of the length of the cord. 



THE SPINAL ANIMAL 



An animal possessing only a spinal cord contains a reflex neural 

 apparatus which can be excited to activity by impulses of various 

 qualities and from any part of the skin. Thus the afferent impulse 

 may correspond to what in ourselves we call tactile and be provoked 

 by mechanical stimulation, or may result from changes of tempera- 

 ture and correspond to those producing sensations of heat and cold. 

 Strong stimuli of any kind may give rise also to afferent impulses which 

 in the intact animal would have the quality of pain. Since these 

 stimuli are such as to produce injury if continued, they may be named, 

 when applied to the spinal animal, pathic or nocuous. The 

 spatial distribution of the stimulus will determine the situation and 

 number of nerve fibres set into action, so that there will be a great 

 variation in the distribution of the excited neurons of the central 

 grey matter according to the quality, distribution, and intensity of 

 the stimulus. The efferent part of the reflex is provided for by 

 the connection of the anterior cornual cells to the whole skeletal 

 musculature of the body, as well as by the distribution of the axons 

 of the lateral horn-cells to the sympathetic system and through this 

 to the viscera. On the other hand, if the spinal cord be separated from 

 the medulla oblongata and higher parts of the brain, it is deprived of 

 all connection with the most highly elaborated sense-organs of smell, 

 sight, hearing, and equilibration, and also of the important afferent and 

 efferent impulses which pass between brain and viscera through 

 the vagus nerves. In studying the reaction of the isolated spinal 

 cord we are studying a nervous system cut off from its most complex 

 components, but at the same time deprived of the initiation and 

 guidance which it must normally be continually receiving from the 

 higher sense-organs through the brain. A study of the spinal animal 

 will therefore be instructive as a study of the mammalian nervous 

 system in its simplest possible aspect. It will, however, in all cases 

 be the study of an incomplete and maimed system, the incomplete- 

 ness increasing as we ascend the scale of animals in our experimenta- 

 tion, owing to the increasing subordination of the lower to the higher 

 centres, and of the immediate reflexes to the educated reactions of 

 the anterior part of the brain. 



