MEDULLA OBLONGATA, the brain stem, or that part of the nervous system thy lies between the pans, or pans Varolii, at th! tipper end, and the spinal cord proper, wilS which it is continuous, at its lower end. It very old part of the nervous system, judo' from the standpoint of evolution, and in it are located the important centres of most of the cranial nerves, including the centres that govert the nerves of the heart and of respiration. It is a triangular cone-like portion of the nerve axis, about an inch in length and one-half to three-quarters of an inch thick, being tubular below and flattened above. Below, it closely resembles the cervical cord in its internal con struction, but above great changes take place to accommodate important new structures. One of the most conspicuous features of the medulla consists in a triangular enlargement of the cen tral canal of the cerebro-spinal axis, constitut ing what is known as the fourth ventricle. The lower seven cranial nerves have their apparent origin in the medulla, and the first spinal nerve also originates in the medulla proper. The internal anatomy of the medulla is complicated. At the lower end the chief motor tracts, the pyramidal tracts that come from the motor area (q.v.) in the brain cross one another to
pass down on opposite sides of the spinal cord. It is because of this decussatiun that a hem orrhage of the brain of the right side causes a paralysis of the muscles of the left side of the body, and vice versa. Just above the decussa tion of the pyramids the end-station. (nuclei) of the two chief sensory tracts of the body are located. It is into these nuclei — the gracilis and the cuneatus — that all of the sense-im pressions from the body below the head are collected, to be passed to higher cerebral centres through the lemniscus or sensory fillet. The chief gray masses in this part of the brain are the nuclei of the cranial nerves referred to, and of the olives, two structures of undetermined function. The best description of the oblongata at present is that of the model of the medulla and pons by Dr. Florence Sabin, which is figured in Barker's \croons System) Con sult also Buck, (Reference Handbook of the Medical Sciences,' article (Train,* and Johns Hopkins Hospital Reports, Welch Memorial Volume. See BRAIN.