322 REGULATION BY NUTRITION. [BOOK i. 



tives, such as sugar for instance, present in the serum have to be con- 

 sidered. We may in addition call to mind, what we said in treating 

 of the skeletal muscles ( 86), that fatigue or exhaustion may have 

 a double nature, the using up of contractile material on the one 

 hand and on the other hand the accumulation of waste products; 

 and the nutritive or restorative influence over the heart of any 

 material may bear on the one, or the other of these. Thus the 

 beneficial effect of alkalies is probably in part due to their 

 antagonizing the acids which as we have seen are being constantly 

 produced during muscular contraction. But we shall return to 

 this subject in dealing at a later part of this work .with the 

 nutrition of the several tissues. 



In the various experiments which have been made in thus 

 feeding hearts with nutritive and other fluids two facts worthy of 

 notice have been brought to light. 



One is that various substances have an effect on the mus- 

 cular walls, apart from the direct modification of the contractions. 



The muscular fibres of the heart over and above their rhythmic 

 contractions are capable of varying in length, so that at one time 

 they are longer, and the chambers when pressure is applied to 

 them internally are dilated beyond the normal, while at another 

 time they are shorter, and the chambers, with the same internal 

 pressure, are contracted beyond the normal. In other words, the 

 heart possesses what we shall speak of in reference to arteries as 

 tonicity or tonic contraction, and the amount of this tonic contrac- 

 tion, and in consequence the capacity of the chambers, varies 

 according to circumstances. Some of the substances appear to 

 increase, others to diminish this tonicity and thus to diminish or 

 increase the capacity of the chambers during diastole. This of 

 course would have an effect, other things being equal, on the 

 output from the heart and so on its work ; and indeed there is 

 some evidence that the augmeritor and inhibitory impulses may 

 also affect this tonicity, but observers are not agreed as to the 

 manner in which and extent to which they may thus act. 



Another fact worthy of notice is when the heart is thus arti- 

 ficially fed with serum, or other fluids or even with blood, the beats, 

 whether spontaneous or provoked by stimulation, are apt to become 

 intermittent and to arrange themselves into groups. This intermit- 

 tence is possibly due to the fluid employed being unable to carry on 

 nutrition in a completely normal manner, and to the consequent 

 production of abnormal chemical substances; and it is probable that 

 cardiac intermittences seen during life are in certain cases thus 

 brought about by some direct interference with the nutrition of the 

 cardiac tissue and not through extrinsic nervous impulses descend- 

 ing to the heart from the central nervous system. Various chemical 

 substances in the blood, arising within the body or introduced as 

 drugs, may thus affect the heart's beat by acting on its muscular 

 fibres, or its nervous elements, or both, and that probably in various 



