. Kirkes' handbook of physiology . omponent segments of the cord. The tracts of the cord arein a sense connectives from segment to segment, connecting the cells ofboth adjacent and of widely separated segments. The function of the cordis comprised in the function of the segments and in the function of the tracts. From a physiological point of view, it may almost be considered as anaxiom that before a nerve cell can send out a nerve impulse it must firstreceive a stimulus of some kind. This stimulus usually consists of an afferentimpulse from the periphery. Its effect upon the receiving cell ma
. Kirkes' handbook of physiology . omponent segments of the cord. The tracts of the cord arein a sense connectives from segment to segment, connecting the cells ofboth adjacent and of widely separated segments. The function of the cordis comprised in the function of the segments and in the function of the tracts. From a physiological point of view, it may almost be considered as anaxiom that before a nerve cell can send out a nerve impulse it must firstreceive a stimulus of some kind. This stimulus usually consists of an afferentimpulse from the periphery. Its effect upon the receiving cell may be insuf-ficient to cause any response, or the response may be delayed for a long period 520 THE NERVOUS SYSTEM and may involve manv complicated nervous activities and even psychologicalprocesses. Where the response is approximately immediate, the reaction isknown as a reflex. A reflex arc, reduced to its simplest terms, consists of the following ele-ments: a, a sensory surface; b, an afferent neurone; c, an efferent neurone; d,. Fig. 364.—Schematic Sketch of a Reflex Arc. A, With two neurones, an afferent and an efferent;B, with three neurones, an afferent, efferent, and a connecting or intracentral neurone. a muscle or gland. The simplest form of reflex arc is schematically shown in figures 364 and 365. The gap between the termination of the afferent neurone and the dendron of the efferent neurone shown in figure 364 is called a synapsis. The reflex arc is probably seldom as simple as that shown in figure 365, where only two neurones are involved. More often, three or moreneurones take part, as shown in figures 364 B,and 366. The neurone connecting the afferentneurone with the efferent neurone belongsto the class of intracentral or connectingneurones. Since all parts of the cord, infact of the entire cerebro-spinal axis, are in-directly connected with one another byintracentral neurones, figure 361, the possi-bility of increasing the number of efferentlimbs of the reflex
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