204 BIOLOGY. [BOOK n. 



to the valves that these seem to be concentrated. Nevertheless 

 sympathetic threads furnished with ganglionaiy expansions 

 penetrate here and there into the muscular substance. 



This structure of the heart is in perfect harmony with the 

 functions of that organ. It is true that this organ belongs to 

 the department of the nutritive life ; but it has to contract with 

 rapidity. Now the smooth muscular fibres of the animal life 

 contract slowly. It was needful then that in animals with a 

 circulation active in a degree, however small, the propulsive organ 

 of the blood should be constituted by striated fibres. Further- 

 more it was indispensable for 'these fibres to have a great 

 functional independence. In effect, the heart, which commences 

 to beat the very first day of the embryonic life, does not definitively 

 stop till death. Contrariwise to what takes place in all the 

 organs of relation and in a considerable number of nutritive 

 organs, the heart has no normal intermittence ; it has no need of 

 reparative repose. It is an indefatigable worker, and the labour 

 it accomplishes during the life of a man is truly prodigious. 

 From birth to death, it can without reposing contract three billion 

 times, and impel into the tissues about 150 millions of kilogrammes 

 of blood. If during sleep its movements are somewhat slower, 

 it is because many organs at that time repose and so expend less. 

 During the hibernal sleep when the nutritive movement falls to 

 the minimum, and when the consumption of oxygen diminishes 

 nineteen-twentieths (marmot), the number of beatings of the heart 

 diminishes nine-tenths. If the heart were strictly enslaved to the 

 nervous system, if it always obeyed it with docility, and waited 

 always for a first, excitation from it, then the heart would share 

 the functional feebleness of the nervous system, which is the 

 most intermittent of the organic systems : but this is far 

 from being the case. The heart severed from the organism 

 still beats for a longer or shorter period according to the 

 species. Thus Scoresby saw the heart of a shark beating 

 some hours after it had been torn from the body (Burdach, 

 vi. 303). The heart commences to beat (punctum saliens) before 



