596 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



From these observations we may conclude that in the dog the motor path- 

 ways follow tracts on both sides of the cord, but that the tract on the same side 

 as the muscles to which it conveys impulses is the more important. If this tract 

 is suppressed by hemisection, the one on the opposite side takes up the function. 

 But the nearer the hemisection lies to the point of exit of any given nerve, the 

 more completely have the fibers destined for that nerve already crossed from 

 the other side of the central system (see page 593) ; consequently the more pro- 

 found is the disturbance. 



When a hemisection is made in the thoracic cord of a dog, and the resulting 



disturbances have disappeared, a hemisection on the other side higher up or a 



sagittal section in the mid line of the cord will produce the same effects again; 



in either case the operation has broken the pathways coming from the heter- 



onymous side. 



But motility is not permanently lost even after a second hemisection like 

 that just described. In fact after three alternating sections at different levels, 

 some motor fibers to the muscles of the hind leg remain uninterrupted (Osawa). 

 The alternative motor efferent pathways therefore must be very numerous. 



Observations on the motor effects of half-destruction of the spinal cord in 

 man may be summarized, after Kocher, as follows: Motor paralysis on the 

 same side appears immediately in a very intense form, but, as a rule, it abates 

 in the course of a few days or weeks, and if the anterior horn nuclei are not 

 too extensively destroyed, it is so far recovered from that only a slight paresis 

 remains. The deep crossing just above the point of exit of the nerves to the 

 extremities is of more importance for the leg than for the arm. 



To judge by the anatomical facts, the motor paths destined for the muscles 

 of a given extremity run in the crossed pyramidal tracts and Monakow's bundle 

 on the same side of the cord and in the direct pyramidal tract of the opposite 

 side. In accordance with this we find it stated that a section involving all 

 parts of the cord except the lateral columns produces only a slight reduction 

 of motility, and that it is not finally abolished by section of the lateral columns 

 alone; in fact, the motility gradually returns and becomes fairly complete 

 again. The above-mentioned observations by Osawa show, however, that there 

 are other efferent paths from the brain to the spinal cord than these. 



We have the following statements concerning the course of the conducting 

 pathways for the vegetative functions. The vasoconstrictor nerves run in both 

 the homonymous and heteronymous paths, the former appear to be the stronger. 

 The tracts to the bladder and rectum are also found on both sides; those of 

 either side being sufficient to innervate the musculature of the entire bladder 

 and the entire rectum. The tract on either side, therefore, can be injured with- 

 out any interference in the function of the bladder or rectum. Finally, the 

 sympathetic fibers to the eye and to the corresponding half of the face descend 

 the whole length of the cervical cord on the same side. Paralysis after destruc- 

 tion of this tract appears to be permanent, although it may decrease gradually 

 in intensity (Kocher). 



2. The Afferent Pathways. Of these the tracts for motor sensations are 



the best known. It has been known for a long time that in certain diseases 



of the spinal cord in man the sense of movement is lost to a greater or less 



> extent without any other loss of sensibility (ataxia, cf. page 472). Patho- 



