Page:Popular Science Monthly Volume 28.djvu/111

Rh 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 per 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 kinæsthetic or sensori motor, and such centers exist in large numbers in the spinal 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 arrangement 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 intimately 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 composed 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