RELATION OF MUSCLES TO THE IIEAKT. 15 



in the vessels themselves were observed microscopically by von 

 Frey and GaskelL* 



The dilatation of muscular vessels on irritation of peripheral 

 nerves was thus brought into a line with the dilatation noticed 

 in the vessels of the submaxillary gland by Bernard, and in the 

 corpora cavernosa by Eckhardt. It is evident that alteration in 

 the size of such a huge vascular tract as the muscular arteries 

 must influence, to a great extent, the blood pressure in tlie 

 arteries generally. It is equally evident that the changes 

 induced in the condition of the blood pressure by muscular 

 action may be of two kinds, either a rise or a fall. If the arte- 

 rioles are compressed by the muscles so that the flow through 

 them is impeded, the general blood pressure will rise. When 

 this effect is more than counteracted by the dilatation of the 

 arterioles themselves under nervous influence, the general blood 

 pressure will fall, for the blood will find an easy passage from 

 the arteries into the veins through the muscles. We can thus 

 see how readily a rise or fall in the general blood pressure may 

 be induced by exercise of the muscles. If they contract suddenly 

 or violently they will tend to compress the arterioles, and raise 

 the blood pressure, whilst if they contract gently their contrac- 

 tion will have little effect in compressing the arterioles, and 

 these, becoming dilated, will allow the blood pressure to fall. 



But there is still another factor v/hich may tend to increase 

 the blood pressure during severe muscular exertion, viz., a 

 quickened pulse, for stimulation of the nerve fibres passing 

 from the muscles to the central nervous system greatly accele- 

 rates the beats of the heart. In this respect stimulation of the 

 muscular nerves differs from that of the cutaneous and visceral 

 nerves, inasmuch as the latter tend rather to slow than to 

 quicken the pulse. This peculiar effect of the muscular nerves 

 upon the heart would, indeed, appear to be a provision of nature 

 for the purpose of maintaining an exceedingly active circula- 

 tion during the active calls upon nutrition which violent exer- 

 tions entail. Muscular exercise, therefore, has a special tendency 

 to raise the blood pressure in the arterial system, and conse- 



* Von Frer, Ludwig's Arheiien, liter Jalirg., 187f), p. lOG ; and Gask .11, 

 ibid., p. 79; and Centralb.f. die Med. IFiss., 1876, p. 557. 



