226 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the impulses travel to the many series of central cells which are concerned in 



the simplest voluntary responses. 



For the most complex voluntary reactions the entire central system is 



necessary, and especially the cortex of the cerebral hemispheres, while it has 



already been shown that the impulses 

 which cause reflex actions can make 

 their circuit in a very limited portion 

 of the spinal cord. In the case of 

 voluntary reactions the impulses take 

 a longer pathway and involve a larger 

 series of central nerve-elements, since 

 from the point at which they enter the 

 system they must pass to the cephalic 

 end and back again to the efferent 

 elements. At the same time, in a 

 voluntary reaction, a greater number 

 of impulses combine to modify the 

 discharge from the efferent cells. 



In order that the encephalon may 

 be included in the pathway of the im- 

 pulses entering the cord, it is necessary 

 that pathways formed by axones 

 should, on the one hand, extend up 

 to the encephalon, and, on the other, 

 back from it to the cord. 



Fig. 96 indicates how the first 

 part of this path is composed of the 

 afferent elements of the dorsal spinal 



fig. 9G.-schema showing the smaller pathway nerve-roots. The long paths in the 



of the sensory impulses, on the lett side, s, S', dorsal funiculi of the cord are formed 



represent afferent spinal nerve-fibres ;C, an afferent ... , _ . _ 



cranial nerve-fibre. This fibre in each case termi- by the ascending branches of the af- 



nates near a central cell, the axone of which ferCnt axoneg an( J t l 1( , S( , terminate, for 



crosses the middle line and ends m the oppo- ' 



site hemisphere. The interruption of the larger the most part, about the cell-bodies 



pathway in the thalamus is not indicated (van \ • \ r t,\ 1 • C j.1 1 1 



, , e] ,,„.,,',,.„, winch form the nuclei of the dorsal 



funiculi at the junction of the cord 

 and bulb. From these nuclei a second series of axones passes out, decussates 

 al ->nce, and then the axones pass forward in the medial lemniscus to rind 

 a second ending in the ventral cell masses of the thalamus, or possibly to 

 continue up to the cortex. From this point a third group of neurones, with 

 their cell-bodies in the thalamus, send out their axones to the cerebral cortex. 



The cranial afferent nerves, which are not nerves of specific; sensations 

 (i. e., the fifth, the vestibular portion of the eighth, the ninth, and tenth), 

 probably have corresponding connections in the central system. 



The impulses which are brought in by the afferent fibres also pass, in a 

 large measure, to cells in the dorsal horn of the spinal cord by way of the 



