320 



MANUAL OF PHYSIOLOGY. 



of the lower part of the spinal cord causes renewed vasomotor 

 paralysis. In frogs this is very well marked, the centres being 

 less confined to the medulla than is the case in the more highly 

 organized animals. 



During recent times numerous investigations on the subject 

 of vasomotor nerves have, no doubt, thrown much light on the 

 subject, but these inquiries have not made the nerve mechanism 

 by which the various vascular areas are governed so clear or so 

 obvious as might be wished. 



Fin. 143. 



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Kymographic tracing showing the effect on the blood- pressure curve of stimulating 

 the central end of the depressor nerve in the rabbit. The recording surface moving 

 from left to right. (c) Commencement and (o) cessation of stimulation. There is con- 

 siderable delay (latency) in both the production and cessation of the effect. (T) Marks 

 the rate at which the recording surface moves, and the line below is the base line. 

 (Foster.) 



In order to explain and reconcile the various experimental 

 truths on this subject (too numerous to be mentioned here), we 

 must suppose that the vasomotor nerve mechanisms are very 

 complex. The supposition of some such arrangements as the 

 following may help to simplify the matter in some degree to the 

 student. 



1. The blood vessels have muscular elements which, though 

 commonly controlled by nerves, are capable of automatic activity. 



