492 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



This evidence, together with the probability that the neuraxons of all the 

 spinal vaso-motor cells end in sympathetic ganglia, 1 makes it fairly credible 

 that the sympathetic vaso-motor nerve-cell possesses central functions. 



There has been much discussion over the meaning of the rhythmic con- 

 tractions observed in certain blood-vessels apparently independent of the cen- 

 tral nervous system. 2 The median artery of the rabbit's ear, the arteria 

 saphena in the same animal, and the vessels in the frog's web and frog's mes- 

 entery, slowly contract and relax. This rhythmic contraction is easily seen in 

 the ear of a white rabbit. The movements are possibly of purely muscular 

 origin, but are more probably the result of periodical discharges by vaso-motor 

 nerve-cells. 



Rhythmical variations in the tonus of the vaso-constrictor centres are often 

 held to explain the oscillations seen in the blood-pressure curve after the 

 influence of thoracic aspiration has been eliminated by opening the chest and 

 cutting the vagus nerves. These oscillations are of two sorts. In the one, 

 the blood-pressure sinks with every inspiration and rises with every expiration, 

 though the rise and fall are not precisely synchronous with the respiratory 

 movements ; in the other, the so-called Traube-Hering waves, the oscillations 

 embrace several respirations. It has also been suggested that these phenomena 

 are due to periodical changes in the respiratory centre affecting the vaso-con- 

 strictor centre by "irradiation." 3 



Vaso-motor Reflexes. The vaso-motor nerves can be excited reflexly by 

 afferent impulses conveyed either from the blood-vessels themselves or from 

 the end-organs of sensory nerves in general. The existence of reflexes from 

 the blood-vessels may be shown by Heger's experiment. Heger 4 observed a 

 rise of general blood-pressure with a subsequent fall, and at times a primary 

 fall, after the injection of nitrate of silver into the peripheral end of the crural 

 artery of a rabbit. The limb, with the exception of the sciatic nerve, was 

 severed from the trunk. The quantity injected was so small that it probably 

 was decomposed before passing the capillaries or escaping from the blood- 

 vessels. Thus the effect exerted by the nitrate of silver on the general blood- 

 pressure was probably caused by afferent impulses set up in the blood-vessels 

 themselves and transmitted through the sciatic nerve to the vaso-motor cen- 

 tres. Vaso-motor reflexes are, however, much more commonly produced 

 by the stimulation of sensory nerves other than those present in the blood- 

 vessels. 



The reflex constriction or dilatation 5 appears usually in the vascular area 



1 See the statement of Langley's results with the nicotin method on page 500. 



2 Literature: Schiff, 1854, p. 508; Mosso, 1880, p. 66; Pye-Smith, 1887, p. 48; Fredericq, 

 1887, p. 351 ; Konow and Stenbeck, 1889, p. 406. Discussion of the active dilatation of the 

 blood-vessels lias been recently revived by Piotrowski, 1892, p. 701 ; Griinhagen, 1892, p. 829; 

 Franck, 1893, p. 729; Biedl, 1894; Stefani, 1894, pp. 237, 245; Lui, 1894, p. 416; Goltz and 

 Ewald, 1896, p. 396. 



3 Compare Fredericq, 1882, p. 71; Knoll, 1885, p. 439. 4 Heger, 1887, p. 197. 



5 For a study of reflex constriction and dilatation produced by stimulating the skin see 

 Maragliano and Lusona, 1889, p. 246; compare Hegglin, 1894, p. 25. 



