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MEDULLA OBLONGATA. The Medulla Oblongata is the expanded portion of the upper part of the spinal cord. It is pyramidal in form and measures one and a half inches in length, three-quarters of an inch in breadth, half an inch in thickness, and is divided into two lateral balves by the anterior and posterior median fissures, which are continuous with those of the cord. Each
VIEW OF CEREBELLUM IN SECTION, AND OF FOURTH VENTRICLE, WITH THE
NEIGHBORING PARTS. (From Sappey.) 1. Median groove fourth ventricle, ending below in the calamus scriptorius, with the
longitudinal eminences formed by the fasciculi teretes, one on each side. 2. The same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle. 3. Inferior peduncle of the cerebellum, formed by the restiform body. 4. l'osterior pyramid : above this is the calamus scriptorius. 5. Superior peduncle of cerebellum, or processus e cerebello ad testes. 6, 6. Fillet to the side of the crura cerebri. 7,7. Lateral grooves of the crura cerebri. 8. Corpora quadrigemina.- After Hirschfeld and Leveille.
half is again subdivided by minor grooves, into four columns, viz.: anterior pyramid, lateral tract and olivary body, restiform body and posterior pyramid.
1. The anterior pyramid is composed partly of fibers continuous with those of the anterior column of the spinal cord; but mainly of fibers derived from the lateral tract of the opposite side, by decussation. The
united fibers then pass upward through the pons Varolii and crura cerebri, and for the most part terminate in the corpus striatum and cerebrum.
2. The lateral tract is continuous with the lateral columns of the cord; its fibers in passing upward take three directions, viz.: an external bundle joins the restiform body, and passes into the cerebellum; an internal bundle decussates at the median line and joins the opposite anterior pyramid ; a middle bundle ascends beneath the olivary body, behind the pons, to the cerebrum, as the fasciculus teres.
The olivary body of each side is an oval mass, situated between the anterior pyramid and restisorm body; it is composed of white matter exter. nally and gray matter internally, forming the corpus dentatum.
3. The restiform body, continuous with the posterior column of the cord, also receives fibers from the lateral column, As the restiform bodies pass upward they diverge and form a space, the 4th ventricle, the floor of which is formed by gray matter, and then turn backward and enter the cerebellum.
4. The posterior pyramid is a narrow, white cord bordering the posterior median fissure ; it is continued upward, in connection with the fasciculus teres, to the cerebrum.
The Gray Matter of the medulla is continuous with that of the cord. It is arranged with much less regularity, becoming blended with the white matter of the different columns, with the exception of the anterior. By the separation of the posterior columns, the transverse commissure is exposed, forming part of the floor of the 4th ventricle; special collections of gray matter are found in the posterior portions of the medulla, connected with the roots of origin of different cranial nerves.
Properties and Functions.—The medulla is excitable anteriorly, and sensitive posteriorly to direct irritation. It serves (1) as a conductor of sensitive impressions upward from the cord, through the gray matter to the cerebrum; (2) as a conductor of voluntary impulses from the brain to the spinal cord and nerves, through its anterior pyramids ; (3) as a conductor of coördinating impulses from the cerebellum, through the restiform bodies to the spinal cord.
As an Independent Reflex Center.—The medulla oblongata contains special collections of gray matter, which constitute independent nerve centers which preside over different functions, some of which are as follows, viz. :
1. A center which controls the movements of mastication, through afferent and efferent nerves. (See page 63.)
2. A center reflecting impressions which influence the secretion of saliva. (See page 66.)
3. A center for sucking, mastication, and deglutition, whence are derived motor stimuli exciting to action and coördinating the muscles of the palate, pharynx, and esophagus, necessary for the swallowing of the food. The secretion of saliva is also controlled by a center in the medulla.
NERVOUS CIRCLE OF DEGLUTITION. (2d and 3d Stages.) Excitor Palatal branch of 5th pair.
Pharyngeal branches of the glosso-pharyngeal.
Pharyngeal branches of the pneumogastric, derived
from the spinal accessory.
Hypoglossal and branches of the cervical plexuš.
5. A speech center, coördinating the various muscles necessary for the accomplishment of articulation through the hypoglossal, facial nerves, and the ad division of the 5th pair.
6. A center for the harmonization of muscles concerned in expression, reflecting its impulses through the facial nerve.
7. A cardiac center, which exerts (1) an accelerating influence over the heart's pulsations through accelerating nerve fibers emerging from the cervical portion of the cord, entering the inferior cervical ganglion, and thence passing to the heart; (2) an inhibitory or retarding influence upon the action of the heart, through fibers of the spinal accessory nerve running in the trunk of the pneumogastric. The cardio-inhibitory center is in a state of tonic excitement and continuously sending impulses to the heart which exert an inhibitory influence upon its action. It may be stimulated directly by anemia as well as venous hyperemia of the blood-vessels of the medulla and increased venosity of the blood. It is excited reflexly by the stimulation of the central end of the vagus, sciatic, and splanchnic nerves.
8. A vasomotor center, which by alternately contracting and dilating the blood-vessels through nerves distributed in their walls, regulates the quantity of blood distributed to an organ or tissue, and thus influences nutrition, secretion, and calorification. The vasomotor center is situated in the medulla oblongata and pons Varolii, between the corpora quadrigemina