512 TEXT-BOOK OF PHYSIOLOGY. 



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 oj spinal cord; e.g., atrophy of the multipotar cells of the anterior 



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

 root fibers. 



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



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



B. THE SPINAL CORD SEGMENTS AS CONDUCTORS. 



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

 function of which is 



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 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 intrin- 

 sic cells of the gray matter, which constitute such a large part of the antero- 

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

 the cord becomes capable of complex coordinated and purposive reflex 

 actions. 



Spino-encephalic or Sensor Conduction. The nerve impulses that 

 arise in consequence of impressions made on the terminals of the nerves 

 in the cutaneous and mucous surfaces, in the viscera and in the muscles, are 

 transmitted through the dorsal roots of the spinal nerves to the cord. On 

 reaching the cord they are received by nerve-cells, the axons of which pass up- 

 ward to and through the medulla, the posterior part of the pons, the poste- 

 rior part of the crura cerebri, and for the most part to the ventral portion of the 

 thalamus opticus, forming what is known as the spino-thalamic system. 

 On reaching the thalamus they are received by nerve cells, the axons of 

 which pass by way of the internal capsule to the cells of the cortex of the 

 cerebrum forming what is known as the thalamo -cortical system. It is 

 probable however that some fibers from the cord and medulla pass direct 

 to the cortex. When thus transmitted through the cord to the cerebral 

 hemispheres directly or indirectly, they are received by specialized nerve- 

 cells in the cortex and translated into conscious sensations. The sensations 

 thus arising may be divided into special and general sensations. Of the 



