500 



ORGAN SYSTEMS OF MAN 



the heart does enough work in one year's 

 time, as computed by the physicist, to lift 

 its owner nearly 100 miles above the sur- 

 face of the earth. When working at full 



Sinus 

 node 



auricular- 

 ventriculor 

 node 



Fig. 19-9. The sequence of the contraction of the various 

 chambers of the heart is under the influence of the 

 sinus node and the auricular-ventricular node. The 

 beat is initiated in the sinus node, from which it 

 travels over the heart as indicated by the arrows. 



capacity, a normal heart can deliver an 

 amount of blood equivalent to that coming 

 from an ordinary household faucet turned 

 wide open! It is truly a remarkable organ. 



Control of the heart 



The heart muscle will function in an ap- 

 parently normal fashion when removed 

 from the body that houses it. Histologically, 

 cardiac muscle is unlike either smooth or 

 striated muscle (Fig. 4-5). The nuclei lie 

 deeply embedded in the muscle fiber like 

 smooth muscle, but cross-striations are 

 present, much like voluntary muscle. Most 

 important, however, all of the fibers are di- 

 rectly connected to other fibers so that the 

 entire mass of muscle fibers is continuous, 

 forming a syncytium. 



Skeletal and smooth muscles contract 

 only on stimulation by nervous impulses or 

 endocrine secretion. The vertebrate heart, 

 on the other hand, may be removed from 

 the body and if placed in the proper nutri- 



ent fluid at the proper temperature will 

 continue beating for hours. Moreover, the 

 rate of pulsation may be quite normal even 

 though it is isolated from any nervous or 

 hormonal control. This might seem to indi- 

 cate that the heart is not influenced by 

 either of these types of control but a check 

 of one's own pulse under varying conditions 

 of excitation or physical exertion quickly 

 shows that such is not the case. This means 

 only that the intrinsic nature of the heart 

 muscle is to contract in a rhythmic manner. 

 The rate of this beat, however, may be 

 attended by outside factors. This can be 

 demonstrated in an excised heart. When 

 placed in a cold fluid it slows down, 

 whereas in a warmer one it speeds up. 

 Therefore, temperature affects its rate. Cer- 

 tain drugs do the same. 



The manner in which the heart beats has 

 been determined rather precisely in studies 

 of the excised heart. It has been shown, for 

 example, that if the heart is subjected to 

 reduced temperatures in various parts, its 

 rate of beat is changed only when a specific 

 region is cooled. This region lies near the 

 base of the great veins where they enter 

 the right auricle (Fig. 19-9). There is a 

 discrete bundle of specialized tissue lying 

 here called the sinus node or "pacemaker" 

 which is responsible. This is clearly shown 

 by the following experiment. If an excised 

 heart is clamped in the region between the 

 auricles and the ventricles, the former con- 

 tinue their rhythmic beating whereas the 

 latter remain quiet. This demonstrates that 

 impulses pass from the pacemaker through 

 the auricles to the ventricles. We have 

 learned further that in passing from auri- 

 cles to ventricles the impulses travel over a 

 bridge called the auricular-ventricular- 

 node ( Fig. 19-9 ) . In the contraction of the 

 heart as a whole, the auricles beat first, 

 the ventricles second. Nerve pathways in 

 the heart have been traced from the auricu- 

 lar-ventricular-node down through the sep- 

 tum between the ventricles and anteriorly 

 through the muscle, and each beat is syn- 



