File:A text-book of human physiology (1906) (14583676750).jpg

Identifier: textbookofhumanp00tige (find matches)
Title: A text-book of human physiology
Year: 1906 (1900s)
Authors: Tigerstedt, Robert, 1853-1923 Murlin, John R. (John Raymond), b. 1874, tr
Subjects: Physiology
Publisher: New York and London, D. Appleton and Co.
Contributing Library: Columbia University Libraries
Digitizing Sponsor: Open Knowledge Commons

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erve show a reflex constriction or dilation of their vessels. Thusthe vast region innervated by the splanchnics is very easily constricted bystimulation of all possible kinds of sensory nerves. It may also be dilatedas the result of a sensory excitation. The vessels of the skeletal muscles as arule appear to be dilated by sensory impulses. The dilation appears first inthe muscles which stand in close functional relationship with the nerves stimu-lated ; but it may be called out also by excitation of distant afferent nerves. If these reflex effects are not confined within too small a vascular field,they influence the general blood pressure. Since in almost every afferent stimulation, vessels somewhere are con-stricted and others dilated, it is evident that changes of pressure may takeplace in both the po. itive and the negative direction. As a rule, afferent excitation produces a reflex rise of pressure (Fig. 97).Under certain circumstances a fall is obtained instead, for example: when
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Fig. 97.—Reflex rise of blood pressure in the rabbit (to be read from riglit to left). skin was stimulated. At a the the afferent nerve stimulated is subjected to the cold; when after having beensectioned it is allowed to regenerate to a certain stage; if the stimulus isweak. At present it is impossible to decide whether these different reflexes arecaused by two kinds of efferent nerves or by the difference in l^ehavior of thevasomotor center to stimuli of different strength. There are nerves howeverwhich, so far as our present information goes, mediate only a fall of pressure,whatever the strength of stimulus. Such a nerve is the depressor, already THE VASOMOTOR CENTERS 237 mentioned at page IJiJ. The afferent nerves from the muscles have the sameinfluence on the blood pressure (Fig. 98). The reflex fall of pressure on stimulation of the depressor appears with bothvagi cut; hence, it is independent of changes in the heart frequency and iscaused essentially by a vasodilation. This i

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