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In reflex action the impulses are mainly through the spinal cord, but to some extent through the bulb, pons, and midbrain. In voluntary action they pass through the cerebrum—a condition that leads to important modifications in the results.
Formulated as the doctrine of monism, it states that the mind and its material basis are merely different aspects of one and the same thing, and that there is only one series of connected elements which are known to us directly as the current of our thoughts and indirectly as the physiological processes going on mainly in the cerebrum.
The spinal cord seems to be a thick mass of nervous matter, but it has a narrow canal at its axis, which passes into the further cerebral ventricles above, and is filled, like these, with a clear fluid. The brain is a large nerve-mass, occupying the greater part of the skull, of most elaborate structure. On general examination it divides into two parts, the cerebrum and cerebellum.
Some have supposed it to be in a small tubercle between the cerebrum and the cerebellum, which is called the pineal gland: in this they have fixed the soul's habitation, because the whole man is ruled from those two brains, and they are regulated by that tubercle; therefore whatever regulates the brains, regulates also the whole man from the head to the heel."
The relation of the cerebrum to cutaneous diseases has been studied much of late, and it is now held that the cutaneous eruptions are mainly due to the degree of inhibiting effect exerted upon the vaso-motor center.
Moving up one step higher, we find that the cerebellum is the organ of equilibrium, and that it as well as the spinal cord operates independently of the conscious will, for no conscious effort of the will is required to make one reel from dizziness. As to the divisions of the greater brain or cerebrum, we want you to note that the lower brain serves a double purpose.
In the vertebrates, in general, the olfactory lobe of the brain is largely developed, much exceeding in size the lobe of the optic nerve. It forms the anterior portion of the cerebrum, and in many instances constitutes a large section of that organ, being marked off from it by only a slight surface depression.
*The Cerebrum.*—The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the cortex. This is made up largely of cell-bodies, and has a grayish appearance.
If a pencil were placed over the ear, what portions of the brain would be above it and what below? Describe briefly the cerebrum, the cerebellum, the midbrain, the pons, and the bulb. Locate and describe the cortex. State purpose of the convolutions. State the general differences between the cranial and the spinal nerves. Locate and give the number of the dorsal-root ganglia.
Of the relations which the cerebellum sustains to the cerebrum and to the different parts of the body, the following view is quite generally held: In the development of secondary reflexes, as already described, conditions are established in the cerebellum, such that given stimuli may act reflexively through it and produce definite results in the way of muscular contraction.
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