File:A text-book of human physiology (1906) (14583748968).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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ent 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 involves primarily the region of theabdominal cavity innervated by the splanchnics, although other parts of the bodymay take part in it. The reflex rise of pressure is produced primarily by a contraction of thevascular region innervated by the splanchnics, even though other regions also
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Tig. 9>^.—Reflex fall of blooil pressure in the rabbit produced by stimulation of an afferentmuscular nerve (to be read from right to left). The period of stimulation is indicated bythe vertical lines. ; 1 = ten seconds. may be concerned. Xot all the vascular regions of the body are constricted, atleast not to the same extent, when the pressure rises; for vasodilatation has oftenbeen observed in dilTerent organs, especially in the muscles. It is difficult to decide in many cases whether a given dilator effect is activeor passive. It may be that with an increase of pressure produced by an extensivecontraction of the splanchnic region, various other regions dilate only becauseof the high pressure. Or it may be that dilatation is actively produced, eitherby a decline in the tonus of the constrictor nerves or by stimulation of thedilator nerves. A fall in pressure obtained reflexly is caused by a reduction of tonus in someof the great vascular regions. But as in the case of a rise of

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