464 A MANUAL OF VETERINARY PHYSIOLOGY 



glion leads to the degeneration of tracts in front of the injury, it 

 is known this must be a sensory or ingoing path ; if the degenera- 

 tion occurs behind the injury, it is evident that the affected 

 fibres have their cell-station farther forward, and that the path 

 is motor or outgoing. Another method of tracing the tracts is 

 the developmental. In the early embryo the fibres have no 

 myelin sheath ; when, later on, this appears, it is observed that 

 all the fibres belonging to the same group are simultaneously 

 invested with myelin. In this way it is possible to determine the 

 fibres possessing a common course. There are other methods 

 of inquiry which need not here be referred to ; sufficient has 

 been said to indicate the nature and difficulties of this class of 

 investigation. 



Paths in the Cord. — The white matter forms paths in the cord 

 which are spoken of as ascending and descending tracts.* 



The ascending and descending tracts in the spinal cord 

 of man have only been mapped out after years of laborious 

 research, in which pathological as well as physiological results 

 have been utilised. In man the tracts are numerous and com- 

 plex ; as the animal scale is descended simplification occurs. The 

 monkey is less complex than man, but more complex than the 

 dog, and so on. Tracts present in man are absent in the lower 

 animals ; for instance, a tract known as the direct pyramidal, 

 which connects some motor centres in the brain with the limbs 

 and muscles concerned, has no representative outside of man 

 and the higher apes. Another, known as the crossed pyramidal 

 tract, and well represented in man and the monkey, is but 

 insignificant in the dog (see Fig. 138). The columns of white 

 matter in the spinal cord of the domesticated animals have 

 not been clearly made out, with an exception to be mentioned 

 presently, and we shall, in consequence, be compelled to refer 

 to the columns in man in order to illustrate the principle 

 on which the work is carried out. The white tracts do not 

 run unbroken throughout the length of the cord ; some are 

 long and others short ; some disappear for good ; others change 

 their relative positions at different levels. Speaking generally, 

 the descending tracts diminish in size from the head towards the 

 tail; the ascending tracts diminish from the tail to the head. 

 The shorter tracts are probably the older ones developmentally, 

 for long tracts are more conspicuous in highly-developed animals 

 in which the independent activity of the cord is imperfectly 

 retained, while short tracts are associated with more indepen- 

 dent function of the spinal cord and less development of the 

 higher centres. 



* These terms have been retained for quadrupeds in preference to the 

 expressions ' head wards ' and ' tailwards,' or ' forwards ' and ' backwards.' 



