The cerebellum is implanted on the upper or posterior aspect of the stem, and we note that it is only about a ninth part of the cerebrum in point of size, and that its substance is not convoluted, but is thrown into leaflets. Until a few years ago the wildest guesses were made regarding the function of the cerebellum. Phrenologists place the centre for amativeness on that part of the head which overlies it. Its exact function is not known, but we are certain it is not the seat of our consciousness, nor of our higher faculties; there is evidence now to show that it is an automatic mechanism for regulating and co-ordinating our muscles when they are in action.
Before the cerebellum and the stem of the brain can be lifted out and shown to us, the anatomist finds it necessary to divide the continuation of the latter as it escapes from the skull to enter the canal of the backbone, where it changes its name and is known as the spinal cord. Even if we did not see the stem of the brain and the spinal cord in their natural position in the body we could tell where the one ended and the other began. In the spinal cord the nerves (31 pairs of them) arise from its sides by two roots, one posterior, the other anterior in position. In the stem of the brain they arise irregularly, some of them corresponding to the line of origin of the posterior roots of the spinal nerves, some to the anterior roots, while others arise in an intermediate line. In 1811, exactly a century ago, a young Scotsman of the name of Charles Bell made a very great discovery relating to the manner in which the nerves arise. While waiting in London for patients, he opened a private school in Soho for teaching medical students the anatomy of the body. He wished to explain to them why the brain was divided into cerebrum, cerebellum, and stem, and why the nerves arose in one manner from the stem and in another manner from the spinal cord. His difficulty was due to the fact that in his day anatomists supposed the brain to be the seat of mind, will, and consciousness. The brain they thought worked as a whole and could despatch or receive a message by any nerve. Bell came to the conclusion that this supposition must be wrong; there must be an explanation for the division of the brain into parts and for the complex manner in which nerves arose. Two reflections put him on the track of his great discovery. Bell did not know what we have seen, that stimulation of part of the brain gave rise to muscular movements, but he did know that there was a nerve track which descended from the cerebrum to the spinal cord through the brain-stem. He guessed it was by this nerve path that the will exercised its dominion over muscles, and he noted that it descended in the front part of the cord葉he part from which the anterior roots arise. He concluded that the anterior roots must also be for carrying messages to muscles. He stimulated the anterior root in a living animal and found the muscles supplied from it were thrown into action. He therefore supposed the anterior part of the spinal cord and stem of the brain must be for producing movements and all the nerves which arise from their anterior parts must be motor nerves. We have already seen that the nerves of sight ended on the posterior part of the stem of the brain. Bell inferred from this fact that the posterior parts of the stem and of the spinal cord must be connected with sensation, for the optic nerve is purely for conveying impressions from the eye to the brain. He inferred that the posterior roots of the spinal nerves which end in the posterior part of the cord must be for conveying impressions from the body to the brain. They are the sensory roots. He stimulated them in living animals, no movements in the muscles were produced. Bell thus made the discovery not only that the roots of a spinal nerve were different in function, but the more important one that the anterior part of the great nerve stem was for motion and that the posterior parts were connected with sensation. We have seen that some nerves arise from the brain-stem neither in line with the posterior nor with the anterior roots but from an intermediate line. Bell inferred they must be peculiar in function because they were peculiar in their origin. Time has proved that in this he was also right; seventy years later Dr. Gaskell, of Cambridge, found that these nerves (ninth, tenth and eleventh pairs of cranial nerves) are different in nature. They supply the heart, lungs and other viscera. They are visceral nerves ; over these the will has no control.
Having thus shown us the chief parts of man's nervous system葉he cerebrum, cerebellum, brain-stem, spinal cord and nerves, our friend the anatomist returns to the part he commenced with葉he cerebral hemisphere 擁n order that we may see some of the peculiar features of the human brain. We wish especially to see that part of the brain which is connected with speech, for of all man's gifts that of language is the most remarkable. A clever parrot or a mynah bird may have a voice and a power of imitation, but they cannot be said to have the power to receive and convey thought; only a human brain has that power. We have already been shown that the convolution of the brain which controls the movements of the lips and tongue is situated in front of the lower end of the central fissure (Fig. 1, p. 24). We might have guessed that the convolution which serves in speech葉he inferior frontal convolution 謡ould be close by. It is a little way further forwards. Its exact position in the head is indicated when a finger is placed on the left temple, about an inch behind the upper margin of the orbit. The inferior frontal convolution is situated on the anterior margin of the deepest depression or fissure of the human brain葉he Sylvian fissure, named after Sylvius, who taught anatomy in Paris during the seventeenth century. It was Broca, working in Paris in the latter half of the nineteenth century, who discovered that the inferior frontal convolution of the left side was connected with speech. The deep fissure of Sylvius, as we have seen, passes backwards and upwards on the side of the cerebral hemisphere; below it is the temporal lobe of the brain in the upper part of which the centre for hearing is situated (Fig. 1). There is a connexion or pathway between the hearing and speech centres, for if we are deaf we must necessarily be dumb; the child listens as it imitates its mother's voice.