THE SPINAL CORD AS A CONDUCTING PATH 603 



nomenon of reinforcement of reflexes. Thus, we may assume that a 

 ''high'' lesion tends to remove the central inhibition and to cause a 

 ''Bahnung" of the reflex circuits, or, that a ''high" lesion gives rise to 

 an increase in the irritability of central parts which in turn induces 

 a similar condition in other divisions of the nervous system. In 

 brief, we may explain this phenomenon either upon the basis of removal 

 of cerebral inhibition or upon the basis of an overflow of irritability 

 from this organ. At all events, the facilitation of the spinal reflexes 

 in consequence of central lesions, finally throws the paralyzed muscles 

 into a state of continued contraction or contracture, their spastic 

 rigidity eventually leading to contortions of the extremities. But 

 a paralysis of the muscles is also present in "low" lesions, because 

 these organs then lose the volitional and tonic impulses from the spinal 

 centers. In the latter case, however, the muscles remain in a perfectly 

 flaccid condition and finally undergo atrophic changes from disuse. 

 These differences in the intensity of the reflexes and in the behavior 

 of the muscles are usually so typical that they may be employed in 

 ascertaining the exact location of the lesion. 



CHAPTER L 



THE SPINAL CORD AS A CONDUCTING PATH— ITS 

 TROPHIC FUNCTION 



The General Structiire of the Spinal Cord. — We have previously 

 noted that the spinal cord in the invertebrates consists of a series 

 of ganglia which severally regulate the activities of those segments 

 of the body to which they have been apportioned. In further develop- 

 ment of this simple reflex system, the different ganglia have been con- 

 nected with one another and with the head-ganglion by means of a 

 system of afferent and efferent fibers which pursue a course parallel 

 to the longitudinal axis of the body. This primitive segmental 

 arrangement is also in evidence in the vertebrates, with this modifica- 

 tion, however, that the reflex functions no longer exhibit a strictly 

 local character but are now more closely correlated and subordinated 

 to the activities of the higher centers. This change necessitates first 

 of all the development of a system of conducting paths which connect 

 the different spinal centers with one another, and fuse them into a har- 

 monious whole. In the second place, it necessitates the formation 

 of certain conducting paths which connect these simple centers with 

 those situated in the brain. In this way, two types of conducting chan- 

 nels have been formed, namely, the short and the long. The former 

 represents the more primitive reflex system over which eventually the 



