THE MOTOR CENTERS AND TEE WILL. 103 



which show that every cycle of nerve-action includes a disturbance of 

 the sensory side as well as the active motor agency. Did we, in fact, 

 admit the possibility of the motor corpuscle acting inr se, and in the 

 absence of any sensory stimulation, we should again be placed in the 

 position of believing that an effect could be produced in the absence 

 of a cause. 



For these reasons such a center has been termed kinsesthetic or 

 sensori motor, and such centers exist in large numbers in the spi- 

 nal cord, and they perform for us the lower functions of our lives 

 without arousing our consciousness or only the substrata of the same. 



But now, turning to the brain, although I am extremely anxious to 

 maintain the idea just enunciated that, when discussing the abstract 

 side of its functions we should remember the sensori-motor arrange- 

 ment of the ideal center, I shall have to show you directly that the two 

 sides — namely, the sensory and motor — in the brain are separated by 

 a wide interval, and that in consequence we have fallen into the habit 

 of referring to the groups of sensory and motor corpuscles in the brain 

 as distinct centers. I trust you will not confuse these expressions, 

 this unfortunately feeble terminology, and that you will understand, 

 although parts may be anatomically separated and only connected by 

 commissural threads, that functionally they are closely correlated. In 

 consequence of the bilateral symmetry of our bodies we possess a 

 double brain — a practically symmetrical arrangement of two intimate- 

 ly connected halves or hemispheres which, as you know, are concerned 

 with opposite sides of the body, for the right hemisphere moves the 

 left limbs, and vice versa. 



For my purpose it will be sufficient if we regard the brain as com- 

 posed of two great collections of gray matter or nerve-corpuscles 

 which are connected with sensory nerve-endings, with muscles, and 

 intimately with one another. 



In this transverse section of a monkey's brain, which is stained 

 dark-blue to show up its component parts, you will see all over the 

 surface a quantity of dark -gray matter, which is simply the richly 

 convoluted surface of the brain cut across. Observe, it is about a 

 quarter of an inch deep, and from it lead downward numerous white 

 fibers toward the spinal cord. The surface of the brain, the highest 

 and most complicated part of the thinking organ, is called the cortex, 

 bark, or rind, and in it are arranged the motor centers I am about 

 to describe. These white fibers coming away from it to the cord, 

 not only are channels conveying messages down to the muscles, but 

 also carrying messages from the innumerable sense-corpuscles all over 

 the body. 



So much for one gray mass of centers. Now, down here at the 

 base of the brain you see two lumps or masses of the same nature, and 

 these are called, therefore, the basal ganglia or gray masses. Since 

 they are placed at the side of the paths from the cortex, and undoubt- 



