THE SPINAL CORD 543 



leg so as to give to the gastrocnemius a slight degree of extension. If 

 the tendon be now sharply struck a quick flexion of the foot is produced. 

 . Ankle clonus. This phenomenon consists of a series of rhythmic con- 

 tractions of the gastrocnemius muscle, varying in frequency from six to 

 ten per second. To elicit this reflex, pressure is made upon the sole of 

 the foot so as to extend the foot at the ankle suddenly and energetically, 

 thus putting the tendo-Achillis and the gastrocnemius muscle on the 

 stretch. The rhythmic movements thus produced continue so long as 

 the tension within limits is maintained. Ankle clonus is never present 

 in health, but is very marked in lateral sclerosis of the cord. 

 . The Toe Reflex. This phenomenon is characterized by a flexion of the 

 foot, then of. the leg and perhaps of the thigh when the great toe is 

 strongly and suddenly flexed. It is present in those diseases of the 

 spinal cord in which there is a pronounced patellar reflex. 

 , The Wrist and Elbow Reflex. These phenomena are characterized by an 

 extension movement of the hand and arm when the tendons of the ex- 

 tensor muscles are sharply tapped. These reflexes are especially 

 marked in primary lateral sclerosis of the cord in the upper portion. 

 The organ reflexes, e.g., the activities of the genito-urinary organs, the 

 tomach, intestines, gall-bladder, etc., which are induced by peripheral 

 timulation have been considered in connection with the physiologic action 

 f these organs. The genito-urinary center is located in the lumbar region 

 >f the spinal cord. In diseased conditions of this region the genito-urinary 

 eflexes are sometimes increased, at other times decreased or even abolished. 

 Direct or Cerebral Excitation. The activity of the emissive cells of 

 he spinal cord segments, due to the arrival of nerve impulses descending 

 he spinal cord from the cerebrum, in consequence of psychic states of a 

 volitional or of an affective or emotional character, will be considered in a 

 ubsequent paragraph entitled " encephalo-spinal conduction." 



3. THE SPINAL CORD SEGMENTS AS CONDUCTORS. 



The white matter of the spinal cord consists of nerve-fibers the special 

 unctions of which are 



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



2. To conduct nerve impulses coming to the cord through afferent nerves, 



directly or indirectly to various areas of the encephalon. 



3. To conduct nerve impulses from the encephalon to the spinal cord 



segments. 



Intersegmental or Associative Conduction. The spinal cord con- 

 sists 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 

 spinal segments it is essential that they should be united by commissura 

 or associative fibers. This is, in fact, accomplished by the axons of the mtr: 

 sic cells of the gray matter, which constitute such a large part of the ant< 

 lateral and posterior root zones. In consequence of this association, 

 the cord becomes capable of complex coordinated and purposive ref 



ome of the nerve-fibers arising in the intrinsic cells beco 

 with the cerebellum through the direct cerebellar tract and the antero-lateral 



