FUNCTIONS OF THE ENCEPIIALON. It will be convenient first to examine the tions of those parts of the encephalon which in structure most nearly resemble the spinal cord.
Functions of the tnedalla oblongata, niesoce phale, corpora striata, and optic thalami.— The medulla oblongata most nearly resembles the cord in form and structure, at the same time that it exhibits most marked and impor tant differences from it. Its subdivisions form connections superiorly with -other parts of the brain, namely, the mesocephale, corpora striata, and optic thalami. These connections are so intimate, that, however convenient it may be to the descriptive anatomist to describe these parts each by itself, it is impossible, in exa mining into their functions, to separate them completely. The functions of one part are so readily affected by a change in any or all of the others, that the effects of experiments are not limited only to the part operated upon, but affect or are affected by the rest. Thus, the olivary columns, vvhich form the central and most essential part of the medulla oblongata, extend upwards through the mesocephale to the optic thalami; and the anterior pyramids form an intimate connection not only with the vesicular matter of the mesocephale, but, to a great extent, with that of the corpora striata. All these parts taken together, with the quadri getninal tubercles, will be found to be the centre of the principal mental nervous actions, and of certain physical actions, which are very essential to the integrity of the economy.
The office of the nerves which arise from this segment of the encephalon throws light upon its function. These nerves are partly destined for respiration, partly for deglutition, and partly also for acts of-volition and sensation.
Destruction of the medulla oblongata is fol lowed by the immediate cessation of the pheno mena of respiration ; and this takes place whe ther it be sitnply divided, or completely re moved. When an animal is pithed, he falls down apparently senseless, and exhibiting only such convulsive movements as may be due to the irritation of the medulla by the section, or such reflex actions as rnay be excited by the ap plication of a stimulus to some part of the trunk.
If, in an animal vvhich breathes without a diaphragm, as in a bird or reptile, the spinal cord be gradually removed in successive por tions, proceeding from below, up to within a short distance of the medulla oblongata, loss of motor and sensitive power takes place succes sively in the segments of the body with which the removed portions of the cord were connected.
But the animal still retains its power of per ceiving impressions made on those parts of the body which preserve their nervous connection with the medulla oblongata, and continues to exercise voluntary control over the movements of those parts. The movements of respiration go on, and deglutition is performed. The higher senses are unimpaired.* These phenomena are sometimes observed in man—in such cases as that alluded to in a former page ; where, from injury to the spinal cord in the neck, below the origin of the phrenic nerve, the patient appears as a living head with a dead trunk. The sensibility and rnotor power of the head are perfect ; respiration goes on partially, and deglutition can be readily performed. The senses and the intellectual faculties remain for a time unimpaired.
Irritation of any part of the medulla oblon gata excites convulsive movements in muscular parts which receive nerves from it, and, through the spinal cord, in the rnuscles of the trunk. Spasm of the glottis, difficulty of deglutition, irregular acts of breathing, result from irritation of the medulla oblongata; and, if the excite ment be propagated to the cord, convulsions will become more or less general.
If a leSion affect one half of the medulla ob longata, does it produce convulsions or paralysis on the opposite side of the body ? This ques tion rnay be certainly answered in the affirma tive, when the seat of the lesion is in the conti nuations of the columns of the medulla oblon gata above the posterior margin of the pons. It is not so easily solved, however, when the disease is situate below the pons. The results of experiment on this subject are contradictory, owing probably to the extreme difficulty of limiting the injury inflicted to a portion of the medulla on one side; and those of Flourens are of no value for the decision of this question, as it appears that he injured chiefly the resti form bodies. Anatomy suggests that a lesion limited to either anterior pyramid would affect the opposite side of the trunk, for it is known that such an effect follows disease of the conti nuation of it in the mesocephale or crus cerebri; and that lesion limited to the posterior half of the medulla on either side would affect the same side of the body, no decussation existing between the fibres of opposite restiform or pos terior pyramidal bodies. The irritating or de pressing influence of the lesion would probably be extended to the spinal grey matter of the same side.