THE AFFERENT PART OF THE NERVOUS SYSTEM 79 



than external. Yet they are not altogether so. Strong 

 pressures acting from without produce effects not limited 

 to the skin, but extending to the underlying muscles and 

 to other tissues. Muscles, tendons, and joints return to 

 the central nervous system a host of impulses, the results 

 of which are effective chiefly in producing adaptive re- 

 flexes, but also to some extent in consciousness. 



The afferent fibers, whatever their origin, enter the 

 spinal cord well around toward its dorsal fissure, and 

 then, as a rule, each divides into an ascending and a de- 

 scending branch. The descending branches are said not to 

 run far before they terminate among the cell-bodies of the 

 spinal gray matter, doubtless by synapses, through which 

 reflexes may be brought about. The ascending branches 

 may run unbroken to the medulla or they may end in the 

 gray matter of the cord. At intervals along their course 

 the branches of both orders have short lateral extensions 

 by which they establish intercourse with other neurons. 

 The arrangement has already been sketched in connection 

 with the discussion of reflexes (see page 50). 



If incoming impulses are to call forth the more elabor- 

 ate reflexes, and especially, we believe, if they are to give 

 rise to sensations, they must reach the cerebrum. The 

 pathway thither is imperfectly known and not of the first 

 importance in a work like the present. It has just been 

 said that some afferent fibers actually reach the medulla; 

 those which have come from the feet and have this 

 anterior reach are the longest of all neurons. There is 

 reason to think that much of the conduction to the brain 

 is accomplished through shorter neurons in a tandem 

 arrangement rather than over these long, uninterrupted 

 channels. When the medulla is attained in either way, its 

 own cells are supposed to take up the task of transmission. 

 These are responsible for the forwarding of impulses to 

 the important intermediate stations which flank the third 

 ventricle and are known as the optic thalami. From these 

 occurs the radiation to the cortex itself. 



Sensations are apparently the psychic accompani- 



