583 



mutual repulsion between the materials of the capillary wall and the 

 elements of the liquor sanguinis, preventing the passage of the latter 

 into the pores of the former, except in so far as they are attracted by 

 the tissues for the purposes of nutrition. 



The heart is believed by the author to be the sole cause of the cir- 

 culation of the blood in the frog's foot, and it is proved experiment- 

 ally that other sources of movement cannot have more than a very 

 trivial influence, and that their cessation, supposing them to exist at 

 all, does not give rise to arrest of the blood or accumulation of cor- 

 puscles in the capillaries. 



Distinct evidences of muscularity and contractility have been de- 

 tected in the veins of the frog's foot, but compared with the arteries, 

 the veins show very little spontaneous contraction. 



Regarding the influence of changes in arterial calibre upon the 

 blood in the capillaries, the author is led to conclude that "the 

 arteries regulate by their contractility the amount of blood trans- 

 mitted in a given time through the capillaries, but neither full dila- 

 tation, extreme constriction, nor any intermediate state of the former 

 is capable per se of inducing accumulation of corpuscles in the latter." 



The influence of the nervous system upon the arteries has formed 

 the subject of a special experimental inquiry, the results of which are 

 given in a supplement to the paper. It is there shown that the con- 

 tractions of the arteries of the frog's web are regulated by a part of 

 the spinal cord, the irritation of which induces complete constriction 

 of the vessels, while its destruction is followed by permanent dilata- 

 tion. Neither stimulation nor removal of the nervous centre for the 

 arteries produces any perceptible change in the quality of the blood, 

 as respects adhesiveness of its corpuscles or otherwise. 



Section III., " on the effects of irritants upon the circulation in the 

 frog's web," commences with an account of some experiments per- 

 formed with tepid water applied for a brief period to the foot. This 

 agent, which was selected as the mildest possible stimulant, produces 

 in a very beautiful manner constriction of the arteries, followed by 

 dilatation, with corresponding changes in the amount of blood trans- 

 mitted through the capillaries, as explained at the close of Section II. 

 When, however, such experiments were frequently repeated upon the 

 same animal, and especially if the temperature of the water was more 

 elevated, effects of a different kind began to show themselves ; the 



