THE SPINAL CORD 549 



of the cord: viz., withdrawal of the inhibitor and controlling influence 

 of the brain. 



The total reflex time may be lengthened by influences which lower the 

 irritability of any one or more portions of the reflex arc. 



The irritability may be lowered by a variety of causes, e.g., 



1. Stimulation of Certain Regions of the Brain. It was discovered by Setche- 



now that when the frog brain is divided just anterior to the optic lobes 

 v Fig. 227) and the reflex time subsequently determined according to the 

 method of Turck, the time can be considerably lengthened by stimula- 

 tion 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 inhibitor in- 

 fluence 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 accom- 

 plished 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 hi a definite period of time, 

 a sensor nerve in a distant region of the body be simultaneously stimu- 

 lated, it will be found that the reflex time will be lengthened or the reac- 

 tion completely inhibited. It would thus appear that when nerve 

 impulses from two different regions of the body are transmitted to 

 the spinal segment one or the other will be effective. Thus, if the 

 one group of nerve impulses is capable of exciting the efferent center, 

 the other group of nerve impulses arising simultaneously may partially 

 or completely inhibit the action of the former and the customary 

 response will be wanting. 



3. Lesions of the Spinal Cord; e.g., atrophy of the multipolar cells of the 



ventral horns of the gray matter; degeneration of the terminals of the 

 dorsal root- fibers. 



4. The Toxic Action of Drugs e.g., chloroform, chloral which are believed 



to exert a depressing action on the nerve-cells themselves. 



THE STRUCTURE OF THE WHITE MATTER 



A transverse section of the cord shows that the white matter completely 

 covers the gray matter except where the dorsal horns reach the surface. 

 Ventrally the white matter of each lateral half is connected by a narrow 

 strip or bridge of white matter, the anterior commissure. Microscopic 

 examination shows that the white matter is composed of vertically olis- 

 posed medullated nerve-fibers which are devoid of a neurilemma. These 

 fibers are supported partly by a framework of connective tissue, and 

 partly by neuroglia. The white matter of each side of the cord is anatom- 

 ically divided into a ventral, a lateral, and a dorsal column or funiculus 

 by the ventral and dorsal roots of the spinal nerves. 



Classification of the Nerve-fibers. From a study of the embryologic 

 development of the white matter and of the degenerative changes which follow 

 its pathologic and experimental destruction, it has been differentiated into 

 a number of specialized tracts which have different origins, destinations, 



