Chap, iv.] THE VASCULAR MECHANISM. 259 



Now, serum is as we have seen a very complex fluid containing 

 several proteids, many ' extractives ' and various inorganic salts. 

 As regards proteids experiments have shewn that peptone and 

 albumose so far from being beneficial are directly poisonous to the 

 heart, that paraglobnlin is without effect, but that serum-albumin 

 will maintain the beats for a long time and will restore the beats 

 of a ' washed-out ' heart. We might infer from this that serum- 

 albumin is directly concerned in the nutrition of the cardiac tissue ; 

 but we are met with the striking fact that a frog's heart may be 

 maintained in vigorous pulsation for many hours, and that a 

 ' washed-out ' frog's heart may be restored to vigorous pulsation by 

 being fed with normal saline fluid to which a calcium salt with a 

 trace of a potassium salt has been added. 1 On the other hand, 

 serum from which the calcium salts have been removed by 

 precipitation with sodium oxalate is powerless to maintain or to 

 restore cardiac pulsations. Obviously in the changes, whatever 

 they may be, through which such fluids as serum, milk and the like 

 (for milk and other fluids have been found efficient in this respect) 

 maintain the beat of the heart, calcium salts play an important 

 part ; and it is tempting to connect this with the relation of calcium 

 salts to the clotting of blood (§ 20). We are not however justified 

 in inferring because serum is ineffective in the absence of calcium 

 salts, that the serum albumin is useless ; and, indeed the beneficial 

 effects of the calcic saline fluid are not so complete as those of serum 

 or of blood ; moreover the possible influences of the various extrac- 

 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 (§ 81), 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. 



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 



1 By Ringer's Heart-Fluid, for instance, which is made by saturating in the cold 

 normal saline solution (65 p. c. sodium chloride) with calcium phosphate, and 

 adding to 100 c.c. of the mixture, 2 c.c. of a 1 p. c. solution of potassium chloride. 



