The medulla oblongata is principally remarkable for its connection with respiration. Respiration is a reflex act in which a stimulus is apparently furnished by the unaerated blood, an impression is conveyed to a nervous centre, and an impulse proceeds thence, producing a co-ordinated movement of the muscles of the chest. No doubt these movements are capable of considerable control by the will, but they are continued in conditions of unconsciousness; and, although by an effort they may be delayed for a moment, the impulse soon becomes imperative, and breaks through all restraint. The centre engaged in this reflex action is the medulla oblongata; and that part of the brain is, therefore, of the utmost importance for the continuance of life. The whole of the rest of the brain may be gradually removed without any interference with respiration; also the spinal cord may be divided at different levels; and only when the section is made high up in the neck, above the origin of the (phrenic) nerves which supply the diaphragm, is respiration materially affected; but when that portion of the medulla oblongata is removed from winch the vagus nerves take origin, respiration ceases at once, and the animal dies. This does not arise from mere interference with the functions of the vagus; for that pair of nerves may be divided, and respiration continues, although, no doubt, the entrance of air into the windpipe is interfered with by paralysis of the larynx, and even when that inconvenience is remedied by an artificial opening into the trachea, death results after a time from the irritation of foreign bodies entering the lungs. The sudden death which follows removal of the upper part of the medulla oblongata is, therefore, only to be accounted for by that part being the centre from which the respiratory movements receive their impulse. It is likewise the centre engaged in the act of swallowing, which, like respiration, continues to be performed after removal of the rest of the brain.

Being the centre which governs respiration, the medulla oblongata is likewise to be regarded as the centre engaged in various spasmodic actions of an occasional kind. In coughing, an irritation of the pneumogastric nerve excites first a spasmodic closure of the glottis, and afterwards a convulsive expiration, by which the air forces its way out at the contracted opening. In sneezing, an irritation of the fifth nerve leads to a convulsive expiration with the glottis open, but the tongue raised so as to divert the expelled air from escaping by the mouth, and send it through the nostrils. In hiccough, the glottis is shut, and a momentary spasmodic contraction of the diaphragm and abdominal walls takes place; while in vomiting, a similar action is more prolonged. The walls of the stomach appear to take no part, or only a secondary part in vomiting; for a dog, in which the stomach was replaced by a bladder, was made to vomit perfectly, by injection of tartar emetic into its veins (Magendie). The excitation of the reflex action in vomiting is not always the same, for it may be the result of irritation of the fauces, or may be induced by nausea, an ill-understood sensation depending on disturbance of the cerebral circulation.

It may be furthur mentioned that irritation of the medulla oblongata in the floor of the fourth ventricle, produces artificial diabetes, that is to say, sugar in the urine. This it does by paralysing the blood-vessels of the liver, and so leading to an abnormal amount of sugar being thrown into the blood.

152. When one of the crura cerebri or optic thalami is divided or destroyed, total paralysis of both sensation and voluntary movement is the result. When the cerebellum is removed, the power of standing, and of all steady and definite movement is lost, although the animal continues to move its limbs in its attempts to stand and walk. It seems as if the impulse to voluntary movement descended from the optic thalami, while the power of co-ordination of movements resided in the cerebellum.

When the corpora quadrigemina are destroyed, total blindness results; and when only one side is destroyed, there is blindness in the opposite eye, a result to be accounted for by the crossing of fibres in the optic commissure. It is curious that injury to the optic thalami appears to have no effect on vision, although the optic tracts arise in part from those bodies, as well as from the corpora quadrigemina.

With regard to the corpora striata, experiment gives none but negative evidence, while the study of development and comparative anatomy show them to be properly considered as part and parcel of the cerebral hemispheres.

It has already been pointed out that in early development the corpora striata make their appearance in the floor of the hemisphere-vesicles; they are covered with grey matter on the surface, continuous with that which lines the whole cylinder of the cerebro-spinal axis; and they have other patches of grey matter within them, which, when cut across, present the striated appearance from which the bodies are named; and the lowest of these is in communication with the island of Reil, so that a communication is here established between the grey matter lining the cerebro-spinal canal, and that of the cerebral convolutions. We have seen also that in different animals, while the corpora striata and cerebral hemispheres are intimately connected, they are very variously proportioned one to the other; for, in fishes, one pair of structures represents both; in the turtle a small corpus striatum lies at the bottom of each hemisphere, looking into the interior of its vesicle; and in birds, the islands of Reil and corpora striata form the greater part of the hemispheres: indeed, in the common fowl, the hemispheres consist of scarcely anything else; and when a physiologist, in vivisection, slices what he terms the hemispheres from a fowl, he in reality, removes in the upper slices the corpora striata covered with a thin membrane representing the roof of the hemispheres.