680 PRINCIPLES OF GENERAL PHYSIOLOGY 



a certain stage it is found that the ventricle only responds to every second 

 or third beat of the auricle. The cause must be at the part clamped, and due 

 to the fact that a second impulse arrives before the tissue has recovered from the 

 previous contraction. It is not easy to see how this could happen if the tissue 

 were merely a conductor of excitation, since mere conduction would not leave the 

 tissue in so pronounced a state of inexcitability. 



Lewis (1915, 1) gives the following summary of the course of the excitation 

 wave in the dog's heart. Starting from the sino-auricular node, it spreads in the 

 auricle in all directions, finally arriving at the auriculo-ventricular node, where it 

 is delayed. It then passes along the bundle of Purkinje tissue and is distributed 

 to all parts of the ventricular muscle by the branches of this tissue, which 

 conducts much more rapidly than the muscle itself. The advantage is a more 

 simultaneous contraction of the whole of the ventricle. The actual conducting 

 tissue consists of striated muscle, containing large amounts of glycogen. The 

 following are the rates of conduction in the three different tissues concerned : 



Purkinje tissue - 3,000 to 5,000 mm. per second. 



Auricular muscle - 1,000 mm. per second. 



Ventricular muscle - - 300 to 500 mm. per second. 



In the toad, Lewis (1915, 2) finds that the excitation spreads along the 

 interior of the ventricle and passes out radially to the surface. The general 

 direction of travel is thus from base to apex, but, at the surface, the extreme base 

 is usually activated somewhat later than the apex. 



ELECTKO-CARDIOQRAM 



The electrical change in the heart muscle has been discussed in the preceding 

 chapter, and it is sufficient to refer here to the value of the electro-cardiogram 

 as a method of investigation. We have seen above how it was used to determine 

 the site of the pace-maker, and it is of equal value in detecting irregularities of 

 transmission from auricle to ventricle, especially as they occur in disease. 



THE CORONARY CIRCULATION 



The hearts of the higher vertebrates have, as would be expected, effective 

 means of supply of oxygen by arteries which arise from the commencement of 

 the aorta. The factors which influence the flow through this " coronary " 

 circulation have been investigated by Markwalder and Starling (1913), using 

 the method of Morawitz of introducing a canula into the coronary sinus. They 

 found that the heart is insufficiently supplied with oxygen if the aortic pressure 

 is lower than about 90 mm. of mercury, an important fact to be remembered in 

 experimental work. The most potent agents in increasing the rate of flow are 

 non volatile metabolites produced by the heart muscle itself in its activity. These 

 substances are present in considerable amount in asphyxia ; in fact, the maximum 

 circulation through the coronary vessels is just when the heart fails. Adrenaline 

 causes dilation, probably by its action in increasing the rate and strength of the 

 beats. The amount of blood flowing through the coronary vessels is very con- 

 siderable, much more than had been supposed by previous workers. We shall 

 have occasion to return to the question of the action of metabolites on blood 

 vessels later. Since their concentration becomes so much greater in asphyxia, it 

 -seems that they are such products of activity as are normally removed in oxidation 

 processes : lactic acid naturally occurs to the mind. The products of partial 

 combustion of carbohydrates, as in Rohde's experiments (page 678 above), may 

 also play a part. We have seen (page 611) that the anaerobic products of cell 

 metabolism, however, are not necessarily the same as the intermediate products 

 of normal oxidation. 



METHODS OF INVESTIGATION 



In recording the movements of the heart cavities, each for itself, the myo- 

 cardiograph of Cushny (1910, 1) is very useful. For determining the intra- 



