. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. The spinal cord is not only the path of conduction of nerve impres- sions from the periphery of the brain and the reverse, but is also the seat of a large number of nervous centres which are capable of acting as reflex centres, or even of originating impulses. The functions of the spinal cord are, therefore, to be con- sidered—first, as a collection of nerve-centres, and, second, as a conductor of afferent and efferent impulses. (a) The Sjnnal Cord as a Collection of Nerve-Centres.— It has been already sta
. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. The spinal cord is not only the path of conduction of nerve impres- sions from the periphery of the brain and the reverse, but is also the seat of a large number of nervous centres which are capable of acting as reflex centres, or even of originating impulses. The functions of the spinal cord are, therefore, to be con- sidered—first, as a collection of nerve-centres, and, second, as a conductor of afferent and efferent impulses. (a) The Sjnnal Cord as a Collection of Nerve-Centres.— It has been already stated that reflex action requires for its performance afferent and effer- ent nerve-fibres and a nerve- centre, and the spinal cord has been mentioned as the main seat of the centres of reflex action. When the spinal cord is divided in an animal, the ap- plication of a stimulus to its skin produces muscular movements of the most diverse kinds, depending, as already indicated, upon the nature, intensity, and the locality of the stimulus. The histology of the spinal cord indicates that, from the direct communication of the posterior roots (which have been found to be paths of con- duction of sensation) through the gray commissure with the anterior roots (which have been found to be the paths of motor impulses), afferent impulses reach the spinal cord through the sensory nerves and are directly conducted to nerve- centres, which again are in communication with motor nerves. It is, therefore, evident that afferent impulses are brought directly to nerve- cells, which again communicate the modified nerve impulse to motor nerves (Fig. 336). In the spinal cord such centres of reflex action may be Fig. 335.—Transverse Section of the Spinal Cord in the Dorsal Region, after Bevan Lewis. (Yeo.) (For references see description under Fig. 333.). Fig. &36.—Section of a Spinal Segment, showing a Unilateral and Crossed Reflex Action. (Landois.) A, anterior, and P, posterior surfaces;
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