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and the calamus scriptorius. The vasomotor fibers having their origin in this center descend through the interior of the cord, emerge through the anterior roots of spinal nerves, enter the ganglia of the sympathetic, and thence pass to the walls of the blood vessels, and maintain the arterial tonus ; they may be divided into two classes, viz. : vaso-dilators, e.g., chorda tympani, and vaso-constrictors, e.g., sympathetic fibers.
Division of the cord at the lower border of the medulla is followed by a dilatation of the entire vascular system and a marked fall of the blood pres
Galvanic stimulation of the divided surface of the cord is followed by a contraction of the blood-vessels and a rise in the blood pressure.
The vasomotor center is stimulated directly by the condition of the blood in the medulla oblongata. When it is highly venous it becomes very active and the blood-vessels throughout the body are contracted and the blood current becomes swifter; sudden anemia of the medulla has a similar effect. This center may be increased in action with attendant rise of blood pressure, by irritation of certain afferent nerve fibers. These are known as pressor fibers. On the other hand, its action may be depressed by other afferent fibers with attendant fall of blood pressure. These are known as depressor fibers.
9. A diabetic center, irritation of which causes an increase in the amount of urine secreted, and the appearance of a considerable quantity of sugar.
10. Respiratory center, situated near the origin of the pneumogastric nerves, presides over the movements of respiration and its modifications, laughing, singing, sobbing, sneezing, etc. It may be excited reflexly by the presence of carbonic acid in the lungs irritating the terminal pneumogastric filaments; or automatically, according to the character of the blood circulating through it; an excess of carbonic acid or a diminution of oxygen increasing the number of respiratory movements; a reverse condition diminishing the respiratory movements.
11. A spasm center, stimulation of which gives rise to convulsive phenomena, such as coughing, sneezing, etc.
12. A center for certain ocular functions, governing the closure of the eyelids and dilatation of the pupil.
13. A sweat center is also localized in the medulla.
NERVOUS CIRCLE OF RESPIRATION (ENTIRELY REFLEX).
Pulmonary branches of the pneumogastric.
Trisacial, or 5th pair.
Nerves of general sensibilty.
Phrenic, distributed to the diaphragm.
PONS VAROLII. The Pons Varolii unites together the cerebrum above, the cerebellum behind, and the medulla oblongata below. It consists of transverse and longitudinal fibers, amidst which are irregularly scattered collections of gray or vesicular nervous matter.
The transverse fibers unite the two lateral halves of the cerebellum.
The longitudinal fibers are continuous (1) with the anterior pyramids of the medulla oblongata, which interlacing with the deep layers of the transverse fibers, ascend to the crura cerebri, forming their superficial or fasciculated portions; (2) with fibers derived from the olivary fasciculus, some of which pass to the tubercula quadrigemina, while others, uniting with fibers from the lateral and posterior columns of the medulla, ascend in the deep or posterior portions of the crura cerebri.
Properties and Functions.--The superficial portion is insensible and inexcitable to direct irritation; the deeper portion appear to be excitable, consisting of descending motor fibers; the posterior portions are sensible but inexcitable to irritation.
Transmits motor impulses and sensory impressions from and to the cerebrum.
The gray ganglionic matter ists of centers which convert impressions into conscious sensations, and originate motor impulses, these taking place independent of any intellectual process; they are the seat of instinctive reflex acts; the centers wlrich assist in the coördination of the automatic movements of station and progression.
CRURA CEREBRI. The Crura Cerebri are largely composed of the longitudinal fibers of the pons (anterior pyramids, fasciculi teretes); after emerging from the pons they increase in size, and become separated into two portions by a layer of dark gray matter, the locus niger.
The superficial portion, the crusta, composed of the anterior pyramids, constitute the motor tract, which terminates, for the most part, in the cor