1. A centre which controls the movements of mastication, through afferent and efferent nerves. (See page 24.) 2. A centre reflecting impressions which influence the secretion of saliva. (See page 25.) 3. A centre for deglutition, whence are derived motor stimuli exciting to action and coordinating the muscles of the palate, pharynx and œsophagus, necessary for the swallowing of the food. NERVOUS CIRCLE OF DEGLUTITION. (2d and 3d Stages.) Excitor or Centripetal Motor or Centrifugal Palatal branch of 5th pair. Pharyngeal branches of the glosso-pharyngeal. Pharyngeal branches of the pneumogastric, derived Hypoglossal and branches of the cervical plexus. Motor filaments of the 3d division of the 5th pair. 4. A centre which coördinates the muscles concerned in the act of vomiting. 5. A Speech centre, coördinating the various muscles necessary for the accomplishment of articulation through the hypoglossal, facial nerves and the 2d division of the 5th pair. 6. A centre for the harmonization of muscles concerned in expression, reflecting its impulses through the facial nerve. 7. A Cardiac centre, which exerts (1) an accelerating influence over the heart's pulsations through accelerating nerve fibres 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 fibres of the spinal accessory nerve running in the trunk of the pneumogastric. 8. A Vaso-motor centre, 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 vaso-motor centre is situated in the medulla oblongata and pons Varolii, between the corpora quadrigemina and the calamus scriptorius. The vaso-motor fibres having their origin in this centre 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 fibres. 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 pressure. 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. 9. A Diabetic centre, irritation of which causes an increase in the amount of urine secreted, and the appearance of a considerable quantity of sugar. 10. A Respiratory centre, situated near the origin of the pneumogastric nerves, presides over the movements of respiration and its modifications, laughing, sighing, 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 centre, stimulation of which gives rise to convulsive phe nomena. NERVOUS CIRCLE OF RESPIRATION (ENTIRELY REFLEX). Excitor or Centripetal Nerves. Motor or Centrifugal Trifacial, or 5th pair. Nerves of general sensibility. Sympathetic nerve. 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 fibres, amidst which are irregularly scattered collections of gray or vesicular nervous matter. The transverse fibres unite the two lateral halves of the cerebellum. The longitudinal fibres are continuous (1) with the anterior pyramids of the medulla oblongata, which interlacing with the deep layers of the transverse fibres, ascend to the crura cerebri, forming their superficial or fasciculated portions; (2) with fibres derived from the olivary fasciculus, some of which pass to the tubercula quadrigemina, while others, uniting with fibres 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 portions appear to be excitable, consisting of descending motor fibres; 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 centres which convert impres sions into conscious sensations, and originate motor impulses, these taking place independent of any intellectual process; they are the seat of instinct. ive reflex acts; the centres which assist in the co-ordination of the automatic movements of station and progression. CRURA CEREBRI. The Crura Cerebri are largely composed of the longitudinal fibres 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, constitutes the motor tract, which terminates, for the most part, in the corpus striatum, but to some extent, also, in the cerebrum; the deep portion, made up of the fasciculi teretes and posterior pyramids and accessory fibres from the cerebellum, constitute the sensory tract (the tegmentum), whieh 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 coordination 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 en 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 fibres from the olivary fasciculus and fibres 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 fibres of the optic nerve. Functions. The Tubercula quadrigemina are the physical centres 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 centres preside over the reflex movements which cause a dilation 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 manifested, as after a section of one crus cerebri, which, however, may be due to giddiness and loss of sight. They also assist in the coördination of the complex movements of the eye, and regulate the movements of the iris during the movements of accommodation for distance. CORPORA STRIATA AND OPTIC THALAMI. The Corpora Striata are two large ovoid collections of gray matter, situated at the base of the cerebrum, the larger portions of which are imbedded in the white matter, the smaller portions projecting into the anterior part of the lateral ventricle. Each striated body is divided, by a narrow band of white matter, into two portions, viz :— 1. The Caudate nucleus, the intraventricular portion, which is conical in shape, having its apex directed backward, as a narrow, tail-like process. 2. The Lenticular nucleus, imbedded in the white matter, and for the most part external to the ventricle; on the outer side of the lenticular nucleus is found a narrow band of white matter, the external capsule; and between it and the convolutions of the island of Reil, a thin band of gray matter, the claustrum; the corpora striata are grayish in color, and when divided present transverse striations, from the intermingling of white fibre and gray cells. The Optic Thalami are two oblong masses situated in the ventricles posterior to the corpora striata, and resting upon the posterior portion of the crura cerebri. The internal surface projecting into the lateral ventricles is white, but the interior is grayish, from a commingling of both white fibres and gray cells. Separating the lenticular nucleus from the caudate nucleus and the optic thalamus, is a band of white tissue, the internal capsule. The internal capsule is a narrow, bent tract of white matter, and is, for the most part, an expansion of the motor tract of the crura cerebri. It consists of two segments, an anterior, situated between the caudate nucleus and the anterior surface of the lenticular nucleus, and a posterior, situated between the optic thalamus and the posterior surface of the lenticular nucleus. These two segments unite at an obtuse angle, which is directed towards the median line. Pathological observation has shown that the nerve fibres of the direct and crossed pyramidal tracts can be traced upward through the anterior two-thirds of the posterior segment, into the centrum ovale, where, for the most part, they are lost; a portion, however, remaining united, ascend higher and terminate in the paracentral lobule, the superior extremity of the ascending frontal and parietal convolutions. The sensory tract can be traced upward, through the posterior third, into the cerebrum, where they probably terminate in the hippocampus major and unciate convolution. Functions. The Corpora striata are the centres in which terminate some of the fibres of the superficial or motor tract of the crura cerebri; others pass upward through the internal capsule, to be distributed to the cerebrum. It might be inferred, from their anatomical relations, that they are motor centres. Irritation by a weak galvanic current produces muscular movements of the opposite side of the body; destruction of their substance by a hemorrhage, as in apoplexy, is followed by a paralysis of motion of the opposite side of the body, but there is no loss of sensation. |