TILE CENTRAL NERVOUS SYSTEM. Some state ments regarding the structure of the nervous sys tem will be found in the beginning of this article. (Consult also the article On NERVOUS SYSTEM AND BRAIN.) At the base of the brain imme diately adjoining the spinal cord we have the medulla oblongata. In this part of the brain are found certain important collections of nerve cells which control the movements of respiration, of the heart. and of the blood-vessels. their activity being entirely unconscious and under the influence of reflex stimulation. Col lections of nerve cells connected with and regu lating, the action of physiological mechanisms are known as nerve centres, and in the medulla we recognize the existence of the three centres men tioned and designate them as the respiratory centre, the cardio-inhibitory centre. and the vaso-motor centre. They and other similar centres regulate the activity of the visceral organs, whose func tions are discharged for the most part without_ our conscious knowledge or control. Injury to the medulla or severance of its connections with the spinal cord is fatal at once, owing to the de struction of the connections of the respiratory centre with the respiratory muscles and the con sequent inability to breathe. In the cerebellum we' have an organ which seems to control the co ordinated activity of the voluntary muscles. Ex tensive injury to the cerebellum, while not ac companied by actual paralysis, renders the ani mal incapable of executing orderly movements. If the animal survives the injury, the ability to make coilrdinated movements may be more or less completely, hut some awkwardness and muscular weakness persist as permanent re sults. In the nerve cells of the cerebral hemi spheres we have the seat of the conscious sensa tions and of all the so-called psychic activities. Modern physiology has shown that different parts of the cerebral hemispheres have different func tions. The optic nerve fibres end eventually in the occipital lobes, and here our conscious sensa tions of vision are mediated. A similar arrange ment pievails for the other senses. In the cortex of gray matter covering the cerebral hemispheres there are in fact certain sense areas each of which mediates a different kind of consciousness. Other areas are connected with the various muscles of the body and form the centres through which our voluntary movements are executed, while in still other regions we have areas in which are stored the memories of past experi ences and in which the factors of sensation are organized into the complex associations that characterize our mental life. Injuries to the brain may, therefore, he followed by quite dif ferent results according to the region involved. There may be a paralysis of this or that group of muscles, a loss of this or that primary sensa tion, or a disturbance or perversion of this or that group of memory associations.. The so-called aphasias form noteworthy illustrations of these facts. Injury to certain limited areas of the brain may result in it partial or complete loss of the power of speech. or of the ability to under stand either written or spoken language without any interference with vision or hearing. When defects of this character exist it is possible to locate the probable seat of the lesion, and not infrequently this knowledge has been made the basis of successful surgical operations.
HistonY OF PHYSIOLOGY. The history of phys iology is coextensive with that of medicine. Its
relatively slow advancement through many cen turies was closely associated with the growth of anatomical. chemical, and physical knowledge. In modern times, however. dating perhaps from the first part of the nineteenth century. phys iology has become an independent science with a 'technique peculiarly its own. yet adopting more and more the exact methods that have arisen in the sciences of physics and chemistry. This de velopment of physiology has been especially marked since investigators abandoned the idea of a special vital force controlling the phenomena of life and set themselves to the task of explain ing the properties of living matter in terms of the forms of energy recognized as responsible for the phenomena of inanimate nature. The great tacts in physiology, as in other subjects. have been discovered in a few cases only by the genius of individual workers. In most instances they have developed gradually as the result of the com bined labors of many investigators, so that a history of the growth of the fnndaniental prin ciples of modern physiology involves a discussion of the progress of the natural sciences in gen oral. Every important advance in physics and chemistry has influenced and will continue to in fluence the development of physiological knowl edge. Some of the significant discoveries that be long especially to physiology and that can be traced more or less exactly to a definite period and individual observers are as follows; The circulation of the blood (Harvey, 1616-28) ; the exi.tenc•e of the lymphatic system of vessels (Aselli. 1622); the microscopic structure of the lungs, capillaries. glamjs, and other tissues (\lal pighi, 1660-90) ; the discovery of the mammalian spermatozoiin (Leeuwenhoek, 1678) ; the meth od of direct measurement of blood-pressure (Hales, 1732); the independent irritability of muscle (Haller, 1757) ; the discovery of oxygen and its functions in respiration (Lavoisier, 1775) the digestive action of gastric juice (Reaumur, 1752) ; the distinction between motor and sensory nerve fibres (Bell and \iagendie, 1811-22); the discovery of the mammalian ovum (Von Baer, 1827) ; the true nature of the secre tions of glands (Johannes Muller, 1830) ; the ex istence of enzymes, unorganized ferments, in the gland secretions and saliva—diastase (Leuehs, 1531) ; pepsin (Schwlm. 1836) ; the nature of reflex actions (Marshall Hall, 1837) ; the elec trical phenomena of muscle and nerve (Du Bois Reymond, 1843); the existence of inhibition and inhibitory nerve fibres (Weber. 1845); the introduction of the graphic method in physiologi cal experimentation (Ludwig, 1847) ; the exist ence of secretory nerve fibres (Ludwig, 1S51); the existence of vasomotor nerve fibres (Ber nard, 1851) ; the existence and functions of gly cogen (Bernard, 1857) ; the function of haemo globin in respiration (Hoppe-Seyler and Stokes, 1862-64) ; the localization of function in the cerebrum (Fritsch and Hitzig. 1S70) ; the estab lishment of the fact of internal secretions (Brown-Sequard and others, 1891). For full information, consult : Foster, Text-Book on Physiology (Philadelphia, 1S85) ; The Ameri can Text-Book of Physiology (2d ed., ib.. 1900) ; Sebiifer, Text-Book of Physiology (New York, 1900). For details of the histological features, consult l'iersol, .4 Text-Book of Normal Histology (Philadelphia, 1901).