If I were asked the difference between the brain and the spinal cord, I should say that the brain is more capable of accumulating impressions, not because of the difference of its substance, but because in it the nerve-cells serving this purpose are found in greater abundance.
The manner in which the brain has formed itself in the evolution of the animal world will render the comprehension of its activities easier. Let us consider the simplest creatures, those possessing, so to speak, only a spinal cord. The nerves branching off from the upper part to the nostrils, eyes, ears, mouth, and elsewhere, were subjected during the long series of generations to more continuous stimuli than other nerves. The cells placed at the roots of these nerves were constantly excited by impressions from the external world; chemical processes and combustion would be more rapid in them, hence the necessity of a more copious flow of blood to those parts which were in greater activity. These cells multiplied rapidly at the roots of the organs of sense, gradually covering a wider field. As the animal structure became more perfect during evolution, and the relations of the animal to the outer world increased, the more abundant and active the cells at the roots of these nerves would become. We must not think here of one individual, although individual exercise does strengthen an organ, but must fix our eyes on the interminable chain of generations, all working in this direction.
It was heredity (by which we still transmit to our children the structure and functions acquired by the nerve-centres) which, through the incessant efforts of our progenitors, enlarged this fertile field, until at last it resulted in the mass of the brain.
If, on visiting a museum of comparative anatomy, the reader will look into the glass cases set apart for the study of the nervous system, he will see that the lowest animals have only a spinal cord, or a very small protuberance at the place corresponding to the brain. As the animal structure becomes more complicated, there is a visible increase of the protuberance, which enlarges gradually the nearer one approaches the superior animals, until at last it reaches its maximum size in man.
II
One of the greatest experimenters of modern physiology, Flourens, had already given it as his opinion that the whole cerebral mass performs the same functions in all its parts, and that if one portion be taken away, those contiguous to it charge themselves with its offices. This affords a partial explanation of the fact that wounds of the brain are far less dangerous than those of the spinal cord. It is always a great wonder, even to us physiologists, every time we convince ourselves on living subjects that the brain is without feeling. Men have been seen who suffered great portions of their brain, which protruded from the skull, to be cut away, and sick drunkards or madmen, who, through the wounds in their head, seized hold of the brain with their hands and destroyed it.
Only in the last few years have physiologists succeeded in preserving alive for some time dogs of which nearly all the convolutions of the brain had been removed. Professor Goltz brought a dog in this state from Strasburg to London, in order to show the phenomena which an animal then presents, at the International Congress of Medicine. I extract a few fragments from Professor Goltz’s work,[9] in order to give an idea of the phenomena exhibited by dogs when deprived of a great part of their brain.
A brainless dog has a stupid, inane look. One reads idiocy even in his eyes. His movements are slow and uncertain. It seems as though he needed far more time than usual to come to a decision. His gait is like that of a goose, there is something inexpressibly strange and comical in it. The animal always walks straight on like an automaton. If he meets another dog, he steps over him if he is little; if he is big he may lift him with his head, or knock him down, but on he goes. He tries awkwardly to step over every object he meets, although by simply stepping aside he might pass on without hindrance. He only finds his dish of food with difficulty, smell guiding him better than sight; he snaps stupidly at everything he sees, even biting his own paws till he howls with pain. He can no longer find the fragments of bone that fall out of his mouth while chewing.
Dogs like these are no longer capable of learning anything, and one might almost say that they have forgotten what they already knew; for instance, they no longer give their paw to their master, as they used to do. Their whole intellectual life is extinguished; only when they hear a knock at the door do they bark, but they always begin too late. Two dogs that hated each other, bit each other when they met, even after both had lost a great part of their brain. Memory diminishes in proportion as larger quantities of the brain are removed, and disappears wholly when nearly the whole organ is wanting.