192 MODIFYING AGENTS. [BOOK i. 



'washed out' quiescent heart be fed in the manner described at 

 p. 179, with diluted blood (of the rabbit, sheep, &c.) it may be 

 restored to functional activity. A similar but less complete resto- 

 ration may be witnessed if serum be used instead of blood ; and a 

 heart fed regularly with fresh supplies of blood or even of serum 

 may be kept beating for a very great length of time. In treating 

 of the skeletal muscles we saw that in their case the exhaustion 

 following upon withdrawal of the blood-stream might be attributed 

 either to an inadequate supply of new nutritive material and 

 oxygen, or to an accumulation in the muscular substance of the 

 products of muscular metabolism, or to both causes combined. 

 And the same considerations hold good for the nervous and 

 muscular structures of the heart, though the subject has not yet 

 been sufficiently well worked out to permit any very definite 

 statements to be made. It seems probable however that an 

 important factor in the matter is the accumulation in the muscular 

 fibres and in the surrounding lymph of carbonic acid, and of the 

 substances which give rise to the acid reaction. 



When the frog's heart is thus 'fed' with various substances 

 the interesting fact is brought to light that some substances, 

 such for instance as very dilute lactic acid, lead to increased 

 expansion, and others, such for instance as very dilute solutions of 

 sodium hydrate, to diminished expansion, or to continued con- 

 traction of the quiescent ventricle. It would appear that the 

 muscular fibres of the ventricle over and above their rhythmic 

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

 they are longer, and the ventricle when pressure is applied to it 

 internally dilates beyond the normal, while at another time they 

 are shorter, and the ventricle, with the same internal pressure is 

 contracted beyond the normal. Further, in the frog at least, when 

 the pause between two beats is lengthened the relaxation of the 

 ventricle goes on increasing, so that apparently the ventricle when 

 beating normally is already somewhat contracted when a new 

 beat begins. In other words, the ventricle possesses what we shall 

 speak of in reference to arteries as tonicity or tonic contraction, 

 and the amount of this tonic contraction, and in consequence the 

 capacity of the ventricle, varies according to circumstances. 



When the frog's ventricle is thus artificially fed with serum or 

 even with blood, the beats, whether spontaneous or provoked by 

 stimulation, are apt to become intermittent and to arrange them- 

 selves into groups. This intermittence is possibly due to the 

 serum or blood 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 have often a similar causation. Various chemical 

 substances in the blood, natural or morbid, may thus affect the 

 heart's beat by acting on its muscular fibres, or its nervous 

 elements, or both, and that probably in various ways, modifying in 



