478 THE NERVOUS SYSTEM. 



so well understood as is that of the motor. But the best opinions 

 may be summarized as follows : 



Tactile and muscular sense-impressions pass up the posterior 

 columns to the nucleus gracilis and nucleus cuneatus, by the 

 internal arcuate fibers and fillet to the optic thalamus, by the 

 posterior part of the internal capsule to the Rolandic area of the 

 opposite side. 



Painful impressions, and those of heat and cold, pass up the 

 gray matter of the cord from cell to cell to the optic thalamus, 

 and by the fibers of the corona radiata to the cortex. 



Afferent impulses reach the cerebellum by Clarke's column, 

 direct cerebellar tract, restiform body, and inferior peduncles. 

 The fibers composing the tract of Gowers have their origin in 

 cells at the base of the anterior cornu, on the opposite side, and 

 its cerebellar fibers pass to the middle lobe by the superior 

 peduncles. 



There are other fibers in the cord, which have their origin in 

 the cerebellum ; but although their existence has been ascertained 

 their destination is not certainly determined, though it is thought 

 by some that they arborize around cells in the anterior cornua. 



As a Nerve-center. Besides the function which the cord per- 

 forms as a conductor of motor and sensory impulses, it also acts 

 as a nerve-center in which, by virtue of its nerve-cells, afferent 

 impulses are received and motor impulses are generated. 



Voluntary motion in the extremities, which motion originates 

 in the brain, is abolished when the cord is divided and its ana- 

 tomic connection with the brain cut off; but there still remains 

 the power of exciting muscular contractions in these muscles, due 

 to the cells of the cord itself. 



Reflex Action. If a frog is decapitated, it has no longer the 

 power of producing voluntary movements ; but if the skin of a 

 foot is irritated by pinching, the foot is pulled away from the 

 source of irritation. This is an instance of reflex action. A slight 

 pinch will cause only the one foot to be withdrawn ; but if it is 

 stronger, the other foot may also be withdrawn. This is known 

 as a spreading of reflexes. Such movements are not spontaneous, 

 but they require the application of a stimulus for their production. 

 The irritation does not act upon the muscles directly, but through 

 the medium of nerves, an afferent nerve carrying the sensory 

 impulse inward to the cord, and an efferent nerve conducting a 

 motor impulse outward to the muscles. If either of these nerves 

 is divided, the action does not take place ; nor does it if the gray 

 matter is broken up. For the performance of a reflex act, there- 

 fore, three things are necessary an afferent nerve, a nerve-center, 

 and an efferent nerve, all in a physiologic condition. 



This can be readily understood by reference to Fig. 275, where 

 h represents the skin, from which passes an afferent nerve to the 



