530 THE NERVOUS SYSTEM 



cerebellar tract on the same side, and thence up through the medulla to the 

 cerebellum, figure 396. The impressions of pain, and of heat and cold, are 

 conveyed to the nerve cells in the posterior cornua of the same side in part, 

 and in part to the nerve cells in the posterior cornu and median gray matter 

 of the opposite side. From this point, the impulses are taken up again by 

 intermediary neurones and conveyed through the anterior and lateral columns 

 of the cord to the brain in the ascending tract of Gowers. By reason of the 

 great number of collaterals and the interpolation in the course of the sensory 

 impulse of many intermediary neurones, it has been difficult to make out very 

 sharply defined tracts in the spinal cord for the conduction of the sensations 

 of temperature, pain, and touch. If one set of fibers is destroyed by disease, 

 others seem able, through the collaterals, to take up its function. We can say 

 that injury to the lateral columns has resulted in loss of the sense of pain, 

 heat and cold, but with only partial disturbance of touch sensations. 



It is probable, also, that pain and temperature sensations cross over at once 

 to a considerable extent and pass up in the opposite side of the cord to which 

 they enter. Touch and the muscle sense impressions, especially the latter, 

 pass up largely upon the same side until they reach the medulla or cerebellum. 



Motor Impulses. Motor impulses are conveyed downward from 

 the cerebral cortex of the brain along the pyramidal tracts, viz., the crossed or 

 lateral, and the direct or anterior, chiefly the former. In the crossed pyr- 

 amidal tract the impressions pass down chiefly on the sids opposite to which 

 they originate, having crossed over in the decussation in the medulla. But 

 some motor impulses do not cross in the medulla, but descend in the direct 

 pyramidal tract to lower levels of the cord, where they cross in the anterior 

 commissure. The motor fibers for the legs partially pass downward in the 

 lateral columns of the same side without decussation. This is also probably 

 the case with the bilateral muscles, i.e., muscles of the two sides that act 

 together, such as the intercostal muscles and other muscles of the trunk. 



It is quite certain, as was just now pointed out, that the fibers of the anterior 

 nerve roots are more numerous than the fibers proceeding downward from 

 the brain in the pyramidal tracts, or the so-called pyramidal fibers. This is 

 because each pyramidal fiber is really a very long nerve process or axone, 

 and is supplied in its course with a large number of collaterals, which go off 

 at different points, and thus put it in relation with different groups of nerve 

 cells in the anterior cornua at various levels. Each nerve fiber of the pyrami- 

 dal tract, by means of its collaterals, can control a number of nerve cells, and 

 can thus coordinate the action of impulses sent out through the anterior roots 

 to a number of groups of muscles. In other words, the gray matter of the 

 anterior cornua contains an apparatus with various complicated coordinating 

 powers, which apparatus is under the regulative control of the neurones whose 

 cells of origin are in the cortex of the brain. This is the same apparatus that 

 is also reflexly influenced by sensory impressions passing to the cord. 



