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 sure. 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. Motor or Centrifugal Nerves. [ Phrenic, distributed to the diaphragm. Intercostals, distributed to the intercostal muscles. 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 consists 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 which 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 pus striatum, but to some extent, also, in the cerebrum; the deep portion, made up of the fasciculi teretes and posterior pyramids and accessory fibers from the cerebellum, constitute the sensory tract (the tegmentum), which terminates in the optic thalamus and cerebrum. Function.—The crura are conductors of motor impulses and sensory impressions; the gray matter, the locus niger, assists in the coördination of the complicated movements of the eyeball and iris, through the motor oculi communis nerve. They also assist in the harmonization of general muscular movements, section of one crus giving rise to peculiar movements of rotation and somersaults forward and backward. CORPORA QUADRIGEMINA. The Corpora Quadrigemina are four small, rounded eminences, two on each side of the median line, situated immediately behind the third ventricle, and beneath the posterior border of the corpus callosum. The anterior tubercles are oblong from before backward, and larger than the posterior, which are hemispherical in shape; they are grayish in color, but consist of white matter externally and gray matter internally. Both the anterior and posterior tubercles are connected with the optic thalami by commissural bands named the anterior and posterior brachia, respectively. They receive fibers from the olivary fasciculus. and fibers from the cerebellum, which pass upward to enter the optic thalami. The corpora geniculata are situated, one on the inner side and one on the outer side of each optic tract, behind and beneath the optic thalamus, and from their position are named the corpora geniculata interna and externa; they give origin to fibers of the optic nerve. Functions.-The Tubercula quadrigemina are the physical centers of sight, translating the luminous impressions into visual sensations. Destruction of these tubercles is immediately followed by a loss of the sense of sight; moreover, their action in vision is crossed, owing to the decussation of the optic tracts, so that if the tubercle of the right side be destroyed by disease or extirpated, in a pigeon, the sight is lost in the eye of the opposite side, and the iris loses its mobility. The tubercula quadrigemina as nerve centers preside over the reflex movements which cause a dilatation or contraction of the iris, irritation of the tubercles causing contraction, destruction causing dilatation. Removal of the tubercles on one side produces a temporary loss of power of the opposite side of the body, and a tendency to move around an axis is mani L |