Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/942

Rh 884 A N A T M Y [ORGANS OF SENSE - do the arteries which supply the abdominal viscera receive their vaso-motor nerves, but the muscular and mucous coats of the stomach, intestines, gall bladder, bile ducts, ureters, and seminal ducts, and the glandular structures of the liver, pancreas, kidneys, spleen, and supra-renal cap sules. It is important also to observe that these plexuses of distribution not unfrequently contain small ganglia, and the branches which supply the muscular coat of the stomach and intestines have minute microscopic ganglia, with stellate nerve cells lying amidst them. The distribu tion of the pneumogastric nerve to the stomach, and its connection with the solar plexus, enables that nerve to stimulate its peristaltic contraction, and, according to some experimenters, that of the small intestine also; but the precise action of the sympathetic on these organs is still a disputed question. The Prevertebral Hypogastric &amp;gt;l?xus is situated in front of the last lumbar vertebra. It receives branches from the lumbar ganglia of the sympathetic, and from the plexus surrounding the abdominal aorta. It divides into two parts, which lie one on each side of the rectum, and are called the pelvic plexuses; these plexuses are joined by branches from the sacral ganglia of the sympathetic, and from the 3d and 4th sacral spinal nerves, and contain small gangliform collections of nerve cells. From the pelvic plexuses numerous plexiform nerves arise, which accompany the internal iliac artery and its branches to the walls and viscera of the pelvis, and are named after them. These nerves not only supply the vaso-motor nerves for these blood-vessels, but also the muscular coat and mucous membrane of the bladder, rectum, and urethra, besides the prostate gland in the male, and the uterus and vagina, and in part the ovary, in the female ; in con-nection with their distribution to these viscera, minute ganglia are found lying amidst the nerves, the nerve cells in which act undoubtedly as centres of reinforcement for the origin of additional, nerve fibres. From the distribution of the branches of the gangliated cord of the sympathetic, and of the gangliated prevertebral plexuses, it will be seen that this nerve is especially related to the blood-vessels and to the viscera. contained within the great cavities of the body. As the cerebro-spinal sys tem is engaged in the supply of nerves to the voluntary muscles, the sympathetic is the medium of supply for the involuntary muscular apparatus, both in the coats of the vessels and in the walls of the hollow viscera. But though the vaso-motor nerves branch from the sympathetic ganglia, it must not be supposed that they have no con nection with the cerebro-spinal system. The communicat ing branches between the sympathetic ganglia and the anterior divisions of the spinal nerves establish a connection between them and the cerebro-spinal nervous axis. By recent experiments, the tract of transmission of the vaso- motor fibres has been traced along with the anterior roots of the spinal nerves, through the lateral columns of the cord to the medulla oblongata, in which the vaso-motor nerve centre lies a little to one side of the mesial plane, above the calamus scriptorius. In the distribution of the sympathetic to the glandular viscera, not only is it im portant to attend to their terminations in the muscular coat of the blood-vessels of the glands, but the termina tion of the nerves in connection with the secreting cells themselves must be taken into consideration. The com munications between the cerebro-spinal and sympathetic systems, not only through the spinal nerves, but also through the pneumogastric, are to be kept in mind in con nection with the effects produced by varying mental con ditions on the secretions of the glands. ORGANS OF SENSE. The organs of sense are the organs through the interme diation of which the mind becomes cognisant of the appear ance and properties of the various objects in the external world. These organs are severally named nose, eye, ear, tongue, and skin. For the excitation and perception of a sensation three sets of structures are necessary : a, a peripheral end-organ ; b, a sensory nerve ; c, a central organ. The peripheral end-organ is the part of the appa ratus to which the stimulus necessary for the production of the sensation is applied. This stimulus causes nervous impulses to be propagated from the end-organ along the fibres of the sensory nerve to the central organ, in which that nerve terminates at its central extremity. These ner vous impulses occasion molecular changes in the nerve cells of the brain, and the mind becomes conscious of a sensation. The shape and construction of each organ of sense is adapted to the application of the stimulus required for the produc tion of the particular sensation to which the organ is sub servient. Each organ of sense possesses its own character istic form of end-organ. The touch corpuscles of the skin, the end bulbs found in several mucous membranes, and the Pacinian corpuscles, are the end-organs occurring in their several localities; they have the peripheral ends of the sensory nerves terminating in their substance, and the axial cylinder of the nerve fibre ends in their interior. The rods and cones of the retina, the rods of Corti in the cochlea, the olfactory cells of the nose, and the gus tatory bodies in the tongue, are the end-organs belong ing to their several organs of sense ; the sensory nerve fibres which terminate in relation with them have not yet, however, been traced into actual continuity with their substance. A stimulus, whatever may be its nature, applied to any organ of sense can excite only that kind of sensation for the production of which the organ is subser vient. Thus a stimulus applied to the eye, whether it be the natural stimulus of the waves of light, the mechanical stimulus of a blow, or an electric stimulus, can only excite the sensation of light. Stimuli applied to the ear can only excite the sensation of sound, and in like manner with the other senses. In studying the anatomy of the organs of sense the arrangement of numerous accessory structures, which assist either in conducting stimuli or in modifiying their effects, the arrangement and structure of the peripheral end-organs, and the origin, course, and distribution of the sensory nerves, will have to be considered. The Noss or organ of smell is a large cavity situated in Nose. the face, between the orbits, above the mouth, and below the cribriform plate of the ethmoid bone. It communicates by the anterior nares, or nostrils, with the external atmo sphere, by the posterior nares with the pharynx, and through it with the larynx, trachea, and lungs. It is the proper entrance to the respiratory passage, is accessory to the pro duction of the voice, aids in the sense of taste, and forms one of the most important features of the face. It is sub divided into a right and a left chamber by a vertical mesial partition, the septum nasi, so that the nose is a double organ in the same sense as the eyes or ears are double. The walls of the cavity of the nose are formed partly of bone and partly of cartilage. The osseous walls are referred to on page 82G. The cartilages form the point, the alae, and a part of the septum nasi. The mesial or septal cartilage is triangular in shape, and fits into the interval between the vomer, the mesial plate of the ethmoid, and the nasal spine of the superior maxilla. Anteriorly and inf eriorly its border is free, projects on to the face, and forms the cohimna of the nose. The lateral cartilages form the tip and alas. On each side is an upper lateral cartilage attached by its