and, if you have only profited by the instruction you have received, you have a store of facts at your command that will enable you to recognize heat, light, electricity, gravitation, magnetism, whenever you see them manifested. When, therefore, you ask me what mind is, I answer that it is a force possessing peculiar properties, and developed by a substance constituting a part of the nervous organism of man and other animals, and known to anatomists and physiologists as gray nerve-tissue. This is similar in all essential respects, so far as its terms are concerned, to the definition that you would give me of any other force. Of course, it can be made more precise and extensive, but no enlargement would change its character.
The gray nerve-tissue exists in the form of aggregations of minute cells in various parts of the nervous system. In man, by far the largest collections are found in the brain, and especially on the outside of it, covering it as the rind covers an orange, and hence called the cortex, or the cortical substance. Besides this large mass, spread out to the thickness of the twelfth of an inch or more over the exterior of the brain, there are masses of gray nerve-tissue in other parts, varying in size from that of a walnut to that of a small pea. In this diagram the situations of the masses of gray tissue existing in the brain are shown. You will observe a very beautiful arrangement for increasing the extent of the cortical substance without at the same time increasing the size of the brain, and thus making it heavier than it would be comfortable to carry. The surface is convoluted, and the gray matter, following the convolutions, is hence doubled over and over again on itself. If the cortex were spread out smoothly, like the skin on an apple, it would cover a body more than four times the size of the average human brain. We should, then, in order to get as much mind-producing substance as we have now, require heads four times the volume of those that we now carry on our shoulders. Gray nerve-tissue is found also in the spinal cord, and some animals, as the frog and the alligator, have more of it in this organ than they have in the brain. It also exists in connection with what is called the sympathetic nerve, in the form of masses called ganglia, and generally placed in intimate relation with the several vital organs of the body—as the heart, stomach, lungs, liver, etc.
Besides the gray nerve-tissue, there is another kind of nerve-substance called the white, and which, instead of consisting of granular forms or cells, is made up of tubes or fibers. The white nerve-substance has nothing to do with the evolution of nerve-force or mind. Its office is to transmit the nerve-force from the places where it is formed to other parts of the body. The great mass of the brain and of the spinal cord, and the whole of the nerves that ramify through the body, consist of white nerve-tissue. You will understand, therefore, that this substance is analogous to the wires of the telegraph, while the gray substance corresponds to the batteries.