130 A MANUAL OF PHYSIOLOGY 



The excitability of the cardiac tissue that is, its power of 

 appropriate response (namely, by contraction) to a suitable 

 stimulus does not particularly concern us here, since it is in no 

 wise a property special to the heart. Only, as we shall see in 

 the sequel, the time-relations of this excitability are of interest, 

 for the existence of a refractory period that is, an interval 

 during which the cardiac muscle refuses to respond to excitation 

 throws light upon the rhythmicity of the heart-beat. The 

 tonicity of the heart i.e., its power of remaining contracted to a 

 certain extent in the intervals between successive beats is 

 another property of great importance in certain aspects, but 

 which only needs to be mentioned at present. 



That the heart-beat is automatic, is sufficiently shown by 

 the fact that, as already mentioned, an excised and empty 

 heart will go on beating for a time, for many hours or even 

 for days in the case of cold-blooded animals. When blood, 

 or even a suitable solution of such inorganic salts as exist 

 in serum, is caused to circulate through the coronary vessels of 

 the excised heart of a warm-blooded animal, it also continues 

 to contract for a long time. But where the cause of the 

 automatism resides, in the muscular tissue or in the intrinsic 

 nervous apparatus, cannot be decided offhand, because in 

 nearly all animals hitherto investigated the muscular tissue, 

 ganglion-cells, and nerve-fibres are inseparably intermingled. 

 In Limulus, however, the horseshoe or king crab, the cardiac 

 ganglion-cells are collected in a nerve-cord running longi- 

 tudinally in the median line along the dorsal surface of the 

 segmented heart, and sending off at intervals branches to two 

 lateral cords, and also branches which enter the heart muscle 

 directly (Fig. 53). When the median nerve-cord is removed, 

 as can be done without injuring the muscle, the heart 

 ceases for ever to beat spontaneously. It still contracts when 

 directly stimulated, mechanically or electrically, but the con- 

 traction never outlasts the stimulation. The automatic power 

 therefore resides in the nerve-cord alone, and not in the muscle. 

 The same is true of the rhythmical power, for excitation of the 

 nerves that pass from the median cord to the muscle produces, 

 ' not a rhythmical series of beats in the resting, and an accelera- 

 tion of the rhythm in the pulsating heart, but a tetanus closely 

 resembling that produced in skeletal muscle on stimulation of a 

 motor nerve ' (Carlson). Conduction and co-ordination are also 

 effected in this heart through the nervous mechanism, and 

 essentially through the median nerve-cord ; for section of the 

 longitudinal nerves in any segment of the heart abolishes the 

 co-ordination of the two ends of the heart on either side of the 

 lesion, while division of the muscle in any segment does not 



