THE SPINAL CORD. , 461 



to the optic lobes and the reflex time subsequently determined 

 according to the method of Tiirck, that the time can be 

 considerably lengthened by stimulation of the optic lobes. 

 This is readily accomplished by placing small crystals of sodium 

 chlorid on the optic lobes. It was concluded from this fact that 

 these lobes contain centers which exert an inhibitory influence over 

 centers in the spinal cord through descending nerve-fibers. 

 This conclusion is strengthened by the fact that division of the 

 brain just behind the optic lobes causes a temporary inhibition 

 of the reflexes in. consequence of a mechanical irritation of these 

 fibers. It is quite probable that the volitional inhibition of 

 certain reflexes is accomplished through the intermediation of 

 this center localized by Setchenow. 



2. Stimulation of Sensor Nerves. If during the application of a 



stimulus sufficient to call forth a characteristic reaction in a 

 definite period of time, a sensor nerve in a distant region of the 

 body be simultaneously stimulated, it will be found that the reflex 

 time will be lengthened or the reaction completely inhibited. 

 The explanation of this phenomenon is not apparent. 



3. Lesions of the spinal cord; e. g., atrophy of the multipolar cells 



of the anterior horns of the gray matter; degeneration of the 

 terminals of the posterior fibers. 



4. The toxic action of various drugs, e. g., chloroform, chloral, 



which are believed to exert their depressing action on the nerve- 

 cells themselves. 



The Spinal Cord as a Conductor. The white matter of the 

 spinal cord consists of nerve-fibers the specific function of which is: 



1. To conduct nerve impulses from one segment of the cord to another. 



2. To conduct nerve impulses from the encephalon to the spinal 



cord segments. 



3. To conduct nerve impulses coming to the cord through afferent 



nerves, directly or indirectly to various areas of the encephalon. 



Intersegmental or Associative Conduction. The spinal cord 

 consists of a series of physiologic segments each of which has specific 

 functions and is associated through its related spinal nerve with a 

 definite segment of the body. For the harmonious cooperation and 

 coordination of all the spinal segments it is essential that they should 

 be united by commissural or associative fibers. This is, in fact, 

 accomplished by the axons of the intrinsic cells of the gray matter, 

 which constitute such a large part of the anterior and posterior root, 

 zones. In consequence of this association, the cord becomes capable 

 of complex coordinated and purposive reflex actions. 



Encephalo-spinal or Motor Conduction. At birth the child is 

 capable of performing all the functions of organic life, such as sucking, 

 swallowing, breathing, etc. It is, however, deficient in psychic 



