Anatomy of the Brain. 571 



raent of the stimulus is downward toward the periphery of the body 

 here, while in the parts pertaining to the sensory tracts it is toward the 

 center, the cortex of the brain. It is evident, therefore, that there is a 

 turning point somewhere, a point at which a stimulus ceases to be a 

 sensor}- stimulus and becomes a motor stimulus. As already remarked, 

 such points exist in all parts of the spinal cord for the generation of 

 the so-called reflex actions. These points are made up of cells, some 

 of which are placed in the posterior columns of the cord and others in 

 the anterior columns, the former being smaller than the latter. The 

 course of the reflex stimulus is inward from the periphery of the body 

 through the ganglion on the posterior nerve root to the small cells of the 

 posterior columns, thence across to the large cells in the anterior col- 

 umns, from which it is reflected down and out again to the periphery. 

 The first half of this course is called the afferent and the sensory, the 

 last half is the efferent and the motor. 



The first deflection of the stimulus is evidently at the posterior cell, 

 the final one at the larger anterior cell. While the result of these two 

 deflections amounts to a 'complete reversal of the direction of the stim- 

 ulus, there is nothing to show any alteration in its nature, the stimulus 

 in reality moving forward throughout its course ; the terms afferent 

 and efferent expressing merely the relative effects on the cells or gan- 

 glia through which it passes. To each and ever}' cell or ganglion 

 through which the stimulus moves, it is in reality both afferent and ef- 

 ferent, as relates to that cell or ganglion. 



The course of those stimuli which are not called reflex, but whose re- 

 sulting actions are denominated voluntary, instead of merely crossing 

 the spinal cord from the posterior cells to the anterior, pass on up the 

 posterior columns of the cord and along the sensory or posterior tracts 

 of the medulla oblongata and crura cerebri to the optic thalamus, and 

 thence to the small cells in the outer strata of the cortex of the cere- 

 brum. Although on this course, ganglia and cells, at various points, 

 may have been aroused and stimulated, the stimulus has been continued 

 through and past them as an ingoing and afferent stimulus till the small 

 cortical cells are reached. Any further progress of the stimulus 

 makes it outgoing, or motor, for having reached the ultimate ganglia 

 there is nothing left for it but return. Its first return delivery is to the 

 large cells in the lower strata of the cortex, these cells apparently bear- 

 ing the same relation to the smaller ones that the large cells in the an- 

 terior columns of the spinal cord do to the small ones in the posterior 

 columns. From these large cortical cells the stimulus is continued down- 

 ward into the anterior parts of the internal capsule, thence on down the 

 anterior fibres of the crura cerebri, the medulla oblongata and the 

 spinal cord, to the motor nerves leading to the muscle or gland to be 

 moved. 



