THE SPINAL CORD AS A REFLEX CENTRE 329 



large number of segments. Analogous methods may be used for tracing the course of 

 the ascending intra-spinal fibres. 



These intra-spinal fibres occur in the following situations : 



(1) In the lateral columns immediately outside the grey matter, in the bay between 

 the anterior and posterior horns. 



(2) Close to the grey matter in the anterior basis bundle. 



($) In the posterior columns, united with the descending branches of the entering 

 posterior roots in the comma tract, and also in the immediate periphery of the cord and 

 abutting on the posterior fissure in the septomarginal tract. 



(4) Mingled with the fibres of the pyramidal tract. 



All these tracts are mixed, i.e. contain both ascending and descending 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 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 temperature 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 (l 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 equili- 

 bration, 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 in all cases be the study of an incom- 

 plete and maimed system, the incompleteness becoming more evident 

 as we ascend the scale of animals in our experimentation, 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. 



