308 



PHYSIOLOGY 



CHAP. 



J 



Some trustworthy observers, on the other hand, have con- 

 tradicted the statements of Kiirschner, Budge, and Pagliani, and 

 hold that the reaction of the heart to a circumscribed stimulus 

 invariably commences in the part directly excited, whence it is 

 propagated either in the peristaltic or in the anti-peristaltic form, 

 i.e. from auricles to ventricle, or from ventricle to auricles. 



On the theory of the myogenic origin of the rhythmic activity 

 of the heart in all animals, both in the embryonic and the adult 

 state, this rhythmicity must be an inherent property of the muscle 

 cells, independent of the agency of the nervous system (whether 

 extra- or in tra- cardiac), which thus fulfils simply a secondary 

 function, regulatory and trophic. Many workers have contributed 

 to the elaboration and stability of this view, among them 

 Engelmann in Germany (1893-97), Gaskell in England (1882-87), 

 and Fano in Italy (1885-90). 



Some of the experimental arguments on which the myogenic 

 hypothesis was founded have been disproved by more recent 

 investigations carried out with better technical methods, which 

 show the presence of nerve-cells in points of the heart at which 

 their non-existence had previously appeared certain. The follow- 

 ing arguments of the myogenists, however, seem incontestable : 



(a) The automatic movements of the embryonic heart begin 

 before the presence of ganglion cells can be demonstrated under 

 the microscope. In the chick, for instance, the heart begins to 

 beat thirty-six hours after incubation, while the ganglion cells are 

 only, formed after six days (His, jun.) ; the human heart begins to 

 beat three weeks after gestation (Pfliiger), while the nervous 

 elements only appear at the commencement of the fifth week (His, 

 jun.). The ganglia or cardiac nerves are not formed in situ by the 

 differentiation of the muscle cells of which the cardiac tube is 

 composed, but enter the heart from outside, from the cerebrospinal 

 and sympathetic systems, penetrating along the veins in the lower 

 vertebrates (fishes, frogs), along the arteries in the higher verte- 

 brates (birds, mammals) (His and Eomberg). 



(&) In studying the cardiac function of the excised embryonic 

 heart of the chick on the second or third day of development, Fano 

 succeeded by the photographic registration of its movements in 

 demonstrating that the beats of the primitive cardiac tube differ in 

 no essentials from those of the adult heart. It exhibits a rhythm 

 in the form of a peristaltic movement that passes from the auricular 

 to the ventricular segment of the tube. The first portion is more 

 resistant, the second is more easily exhausted. If divided by a 

 transverse section, the first continues to beat, the second stops. 

 Longitudinal or oblique sections of the cardiac tube show that the 

 segment nearest the venous end is the first to contract, and from 

 it the contractile wave is propagated towards the arterial 

 extremity. Toxic or indifferent gases first arrest the beats of 



