Inhibition.—When an impulse is sent along a nerve to a nerve center, if the latter be in a quiescent or unoccupied state, it will transmit outwardly to an organ is reflex impulse. A nerve transmitting an impulse toward a center is called an afferent, while one which con veys an outward impression toward an organ, or toward the periphery, is called an efferent nerve. When, however, a nerve center is preoccupied, and an afferent impulse is sent to it, which would ordinarily be reflected along an efferent nerve, it is not so reflected, or only in a much diminished degree. This peculiar action of arresting a nervous current is called inhibition, and is constantly playing a most hnportant part in the distribution of nervous force to all the different parts of the complicated animal mechanism. For example, it the central end—the end toward the brain or, to speak more definitely, toward the medulla oblongata (see BRAIN, ante)—of a divided pneumogastric nerve be stimulated when the respiratory center is engaged in its accustomed rhythmic action of sending out impulses of inspiration and expiration, one of two things will happen, viz.: either the respiratory movements will be slowed or stopped entirely, or they will he accelerated. One explanation of the difference of action is that the pneumo gastric nerve contains among its afferent fibers two sets, which, on accannt of original Structure or of their connections, will, in one case, stop impulses, while in the other they will he accelerated. The retarding or stopping set are called inhibitory, while the others are called accelerating fibers. It must be confessed, that this is simply "explanation." The most striking instance of inhibition is offered by the heart. If an the pneumogastrics be divided in an animal while the heart is heating regularly, and the outer end of the divided nerve is stimulated for a short time with the interrupted (Fara dic)galvanic current, the heart will be almost instantly brought to a stand-still, it will not resume its pulsations for some time. The beats, when they reappear, are at first feeble and infrequent, but soon reach the standard, or even go beyond it. The inhibitory action in this case has been estimated to be about .16 of a Other stimuli than the interrupted current may be employed, such as mechanical irritation and chemical stimuli. The action of is remarkable. When injected into the blood, stimula tion of the pneumogastric nerve with powerful galvanic currents will produce no inhibi tion. In order to understand this subject better it will be proper to say a few words about rose-motor sierra. In mammals, if the cervical sympathetic nerve be divided, it will be found that the minute arteries of the head on the same side have become dilated, mid that an increased amount of blood is circulating in them. if a rabbit be the subject of the experiment it will be seen that the whole ear of the side operated on will be much redder than natural. All the minute vessels, both arteries and veins, become greatly dilated and the temperature will rise more than one degree. Division of the sciatic nerve in the mammal causes a dilation of the arteries of the leg and foot, and the tem perature will rise several degrees. Division of one of the splanchnic nerves (nerves arising from sympathetic ganglia in the thorax) produces dilation of the blood vessels of the intestines and other abdominal viscera, If a nerve supplying a muscle is divided, there immediately follows a considerable increase of flow of blood from the muscle, showing that its blood vessels have become dilated. The act of dilation may be easily seen with the microscope in the frog: There are, indeed, in all parts of the body certain vascular areas, which have such a relation to certain nerves that they become dilated on division of one of the nerves. These nerves are ordinarily called vaso-motor 'tierces. It is not intended by this designation to indicate that they are vaso-motor iu an exclusive possessing essentially different properties or structure from other nerves, but that they are nerves which have a vaso-motor function, because they are distributed to organs where vase-motor functions can be performed. It has been a matter of discussion as to whether the origin of the vaso-motor nerves is in the spinal cord or in the sympathetic system. Front their being so usually traced to sympathetic ganglia it was supposed for a time that they had a sympathetic origin, but careful researches have demonstrated their origin in the cerebro-spinal centers. One remarkable fact in regard to the vaso motor nerves is that, although a section of their fibers produces dilation in the vessels supplied by them, irritation of the ends (peripheral) with these vessels does not always restore their ealibu• by constriction; but dilation may be increased by irrita. tion in some eases, while constriction is produced in others. 1%.
If an animal is placed under the influence of curari (q.v.), and the central end of a divided sciatic nerve stimulated, an increase of blood-pressure will be observed, caused by the constriction of certain arteries, particularly those supplied by the splanchnic nerves; and, in general, the stimulation of a y nerve containing afferent fibers will pro duce increase of pressure in the blood vessels, thus affording a test for the presence of afferent fibers in any nerve. If the animal is placed under the influence of chloral instead
of cuinri, there will be a diminution of blood pressure instead of an increase on stimula tion of an afferent nerve. These experiments suggest that the dilating or constricting results of stimulation depend upon the condition for the time being of the central nervous system, and this has led to the explanation of the two phenomena by means of the inhib itory nervous functions. This explanation, however, seems to fail in those cases where irritation of the peripheral cnd of a divided nerve produces either dilation or constric tion, according to circumstances. Therefore, it has been supposed that there are vaso motor nerves which have a dilating, and others which have a constricting, function. An interesting example is afforded by the submaxillary gland, which is supplied with two nerves, one being distributed to the salivary duct and its branches, and the oilier to the blood vessels. No marked effect is produced by simple section of either nerve, but when they arc stimulated remarkable results follow, the ducts of tho gland becoming greatly dilated, while the arteries are in an equal degree contracted. A question as to whether there is a vaso-motor center has for some time been a subject of investigation. but no definite conclusions have been obtained. Experiments show that the medulla oblongata, which is the center of so many reflex actions, is also a center of a large munber of reflex •aso-motor functions, but it is also found that various parts of the spinal cord are centers of reflex vaso-motor functions: and it is probable that vaso-motor action is connected with an arrairement of nerve fibers and centers similar to that of the general reflex system. Whether there are automatic vaso-motor centers, those in which. i impulses are generated independently of reflex action, has not been determined, but it s probable that there are, if we consider the effects of certain emotions, such as blushing and pallor.
The pnenmoi,rastric nerve (see NERVOUS SYSTEM, (trite) has the most remarkable course, the greatest number of connections, and the most varied functions of any of the cranial nerves. It arises in the medulla oblongata by a purely sensory root, but municates with five motor nerves in its course, and is distributed extensively to muscular tissue both of the voluntary and involuntary kind, to.striated muscles and to non-striated, whether belonging to blood vessels, intestinal walls, Or glandular duets: , presents two ganglionic enlargements in its course—one at its exit from the cranial cavity by the ju gular ioramen, which is called the ganglion o: the root; the other, a grayish enlargement, Irian half an inch to an inch in length, called the ganglion of the trunk. It is a plex iform structure, composed mainly of white and gray fibers and nerve cells. The most important communication, or addition, which the pneumogastric receives is a branch from the spinal accessory, for it has been shown that the larynx is supplied by filaments derived from this branch. Other communications arc received between the root and the ganglion of the trunk, the most important of which are those from the superior cm vital ganglion of the sympathetic. The filaments are short, and bind the sympat bet ic ganglion to the trunk of the nerve. Other filaments are received from other sympathetic ganglia, so that the pueumogastric becomes a mixture of sympathetic and cerebro-spinal nerves. What mysterious purpose is serval by this mingling is not known: probably qualities are given to the nervous influence of certain titters, similar to those which are sometimes produced in galvano-electrie machines by certain combinations of conductors anti mag nets, producing variations in intensity or in quantity, :mil also, perhaps, other qualities are unknown. The most important branches of the sympathetic are: 1, auricular; 2, pharyngeal; 8, superior laryngeal; 4, inferior laryngeal; 5, cardiac; 6. pulmonary; 7, esophageal; 8, abdominal. e pneumogastries are generally alike in their distributh n in either side of the body, with the exception of the inferior laryngeal and andominat branches. Passing by the differences in the former, it may be stated here that the left abdominal branch supplies innervation to the muscular walls and the mucous membrane of the stomach; numerous branches also supplying the liver, in its course anastomosing with branches of the right abdominal and of the sympathetic. The right pncumogastrie, which is situated more posteriorly, sends a few filaments to the stomach, but is princi pally distributed to the liver, spleen, kidneys, supra-venal capsules, and (the most impor tant difference) to the whole of the small intestine. Before the nerves pass to the intes tines, however, there is a free interchange of fibers between the left and right abdominal branches. •