THE SPINAL CORD 541 



the cord becomes exalted. As the disease progresses the irritability 

 increases to such an extent that violent spasmodic contractions of the 

 arms and legs arise when the skin or tendons are mechanically stimu- 

 lated. The explanation offered is practically the same as hi division 

 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 arches. 



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 

 (Fig. 232) and the reflex time subsequently determined according to the 

 method of Tiirck, 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 in 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. 



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 

 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. 

 Special Reflex Actions. Among the reflexes connected with the 

 more superficial portions of the body there are some which are so frequently 

 either increased or diminished in pathologic conditions of the spinal cord 

 that their study affords valuable indications as to the seat and character of 

 the lesions. They may be divided into: 



1. The skin or superficial reflexes. 



2. The tendon or deep reflexes. 



3. The organ reflexes. 



The skin reflexes, characterized by contraction of underlying muscles, 

 are induced by stimulation of the afferent nerve-endings of the skin eg., 

 by pricking, pinching, scratching, etc. The following are the principal skin 



reflexes: . , 



Plantar reflex, consisting of contraction of the muscles of the foot, induced 

 by stimulation of the sole of the foot; it takes place through the seg- 

 ments of the cord which give rise to the second and third sacral nerves. 

 Gluteal reflex, consisting of contraction of the glutei muscles when the : 

 over the buttock is stimulated; it takes place through the segments 



i. 



3- 



giving origin to the fourth and fifth lumbar nerves. 

 Cremasteric reflex, consisting of a contraction of the cremaster muscle 



