The medulla is the central source and regulator of the respiratory movements. It is in a limited portion of this enlargement of the cord, near the origin of the eighth pair of nerves, that, as Flourens has demonstrated, the organ which he calls the prime mover, or vital node, of the respiratory mechanism has its seat This organ, according to M. Longet, does not comprise all the substance of the bulb, but is only a fascicle, composed of gray substance between the pyramidal and restiform bodies. The medulla transmits impressions from the cord to the brain, and the impulse of the will from the brain to the cord; its anterior and posterior portions are prolongations of the corresponding medullary fascicles, and we may conclude therefore that they share their functions as they do their substance; and that the medulla by its anterior portion controls movement, and by its posterior portion sensation. In point of fact, all the nerves which spring from the anterior portion are sensitive, and from the posterior motor. The anterior fascicles of the medulla cross their fibres, and from this results a cross action on the motor nerves, which originate from these fascicles; the posterior fascicles, on the contrary, do not cross each other, and their action is direct.
The movements of locomotion are originated specially, according to M. Longet, in the pons Varolii This portion of the encephalon has a cross action on motion. It is a centre of perception for tactile sensations, but nothing authorizes us to believe that it can appreciate sensation by itself alone, and without the aid of the cerebral lobes.
These organs unite the greater and lesser brain to the isthmus of the encephalon, and to the spinal cord, and seem to be solely devoted to the transmission of motion and sensation. An injury to one of the middle peduncles of the cerebellum causes the body to turn on its axis; a phenomenon which has been variously explained by different writers.
The hypothesis of Descartes has popularized, so to speak, this organ, whose functions are entirely unknown. The illustrious philosopher believes the pineal gland to be "the source from whence the most subtle parts of the blood, the spirits, flow to all parts in the brain, and are directed to a particular point, according as the gland is inclined one way or the other." This idea of Descartes has been parodied, by making the pineal gland the seat of the soul, from whence it directs the impulses of the brain by two nervous prolongations, called the "reins of the mind" (habenae animi).
In spite of the name which has been given them, these portions of the encephalon do not seem to have any appreciable action on the sense of vision; but they act upon the voluntary movements in such a manner that the influence of the right half is felt on the left, and vice versa; this is called cross action, which is caused, as already stated, by the crossing of the cerebral fibres. The optic beds seem to have no influence over the movements of the upper extremities, as has been thought by several physiologists.
It is unnecessary to enumerate the other portions of the encephalon, of which the functions are doubtful, or entirely unknown.