552 Comparative Animal Physiology 



synchronized. Lymph hearts transplanted to the superficial lymph sac of the 

 tongue beat strongly although spasmodically. Posterior lymph hearts (Bufo) 

 have several separate chambers, and when they are transplanted the beat 

 of these chambers may not remain coordinated.^^^* Histological examination 

 failed to reveal nerve cells anywhere among the branched striated muscle 

 fibers. In intact animals acetylchoHne (10"^— IQ-^) may cause reflex in- 

 hibition, but transplanted denervated lymph hearts are accelerated by acetyl- 

 choline (10-'^— 10-^). This is an exception to the rule of inhibition and 

 myogenicity, but lymph hearts are more closely related pharmacologically to 

 skeletal muscle than to systemic hearts.^^-^ Normally the lymph hearts are 

 controlled by the spinal cord, but they can readily revert to myogenic beats 

 which are less regular and less coordinated than their neurogenic beats. 



From the preceding evidence, cardiac muscle of all animals is potentially 

 capable of spontaneous rhythmic contractions. In embryos of vertebrates and 

 of Limulus the heart beats without a fixed pacemaker. In adults the pace- 

 maker becomes localized-specialized muscle in the vertebrate, and ganglion 

 cells in Crustacea, Limulus, and many insects. After removal of the pace- 

 maker, heart muscle may show some contractions, particularly if the heart 

 is distended. Functional distinction between pacemaker nerve cells and sec- 

 ondary neurones of regulating nerves must be made by deganglionation. 

 Embryonic hearts prior to innervation, whether they are to be myogenic or 

 neurogenic, are insensitive or very slightly sensitive to acetylcholine. In- 

 nervated myogenic hearts are inhibited, and innervated neurogenic hearts 

 are accelerated by acetylcholine. Some adult hearts (Arteinia and Euhran- 

 chifus but not amphibian lymph hearts), are by this test myogenic and 

 non-innervated. 



Heart Rates, Statements of absolute heart rates are of little value unless 

 the conditions are stated accurately. There are extensive tabulations of heart 

 rates.^^^' ^'^" In general the heart rate is slower in sluggish animals than in 

 active ones. For example, among rnolluscs, heart rates in clams range from 

 0.2 to 22 per minute, whereas in squid and octopus the heart may beat at 40- 

 80 per minute."" Heart rate in fast-swimming fish exceeds the rate in slug- 

 gish ones. The rate of beat in some hearts, as in Helix, increases many times 

 as the internal pressure increases. 



Heart rate varies with size, age, and state of development. The rate of 

 beat in a fish embryo increases as the embryo nears the time of hatching. 

 Body size in a given class has been stated to be inversely related to heart rate, 

 but it is probable that activity is more important. In Daphnia the heart beats 

 about 150 times per minute, as contrasted with 30-60 beats per minute in a 

 crayfish. The heart rate in large animals such as elephants and horses is 

 25-40 per minute, compared with rates of 300-500 in rats and mice. Many 

 insect hearts are fast, particularly when the animals are active. In the imago 

 of Sphinx the rate is 40-50 per minute when at rest, 110-140 when active.^*''' 

 The heart rate in birds is very high, 150-180 for domestic fowls, and several 

 hundred per minute for smaller birds. 



Temperature is an important variable in determining heart rate. The high 

 rate in birds is in part related to their high body temperature. Heart rate, 

 in general, increases about two to three times for a 10 degree rise in tem- 



