CHAP, iv.] THE VASCULAR MECHANISM. 303 



function of the spinal ganglia alluded to in 100, we may suppose 

 that the cardiac ganglia are in some way concerned in the nutrition 

 of the cardiac nerve fibres. But our knowledge is not yet suffi- 

 ciently ripe to allow exact statements to be made. 



Other Influences regulating or modifying the Beat of the Heart. 



162. Important as is the regulation of the heart by the 

 nervous system, it must be borne in mind that other influences 

 are or may be at work. The beat of the heart may for instance 

 be modified by influences bearing directly on the nutrition of the 

 heart. The tissues of the heart, like all other tissues, need an 

 adequate supply of blood of a proper quality ; if the blood vary 

 in quality or quantity the beat of the heart is correspondingly 

 affected. The excised frog's heart, as we have seen, continues to 

 beat for some considerable time, though apparently empty of blood. 

 After a while however the beats diminish and disappear; and their 

 disappearance is greatly hastened by washing out the heart with a 

 normal saline solution, which when allowed to flow through the 

 cavities of the heart readily permeates the tissues on account of 

 the peculiar construction ( 151) of the ventricular walls. If such 

 a 'washed out ' quiescent heart be fed with a perfusion cannula, in 

 the manner described ( 155), with diluted blood (of the rabbit, 

 sheep, &c.), it may be restored to functional activity. A similar 

 but less complete restoration 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 how- 

 ever that an important factor in the matter is the accumulation in 

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

 and especially 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, that is to continued 

 contraction, 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 



