324 THE CIRCULATION. 



left nipple ; and still further to the left it is heard alone, the first sound 

 at this situation being purely valvular, like the second. 1 



The movements of the heart ma}' be observed in the dog, or other 

 warm-blooded quadruped, by opening the cavity of the chest by a longi- 

 tudinal incision through the sternum, and separating the costal cartilages, 

 on each side, at their junction with the ribs; artificial respiration being 

 maintained by the nozzle of a bellows inserted in the trachea. The 

 animal may be partially narcotized by a preliminary subcutaneous in- 

 jection of morphine, after which complete etherization is produced and 

 continued with great facility. The operation of opening the chest and 

 exposing the thoracic organs increases the rapidity of the heart's move- 

 ments and diminishes their force ; but its action is not otherwise 

 changed, and the circulation will continue for several hours, provided 

 artificial respiration be maintained with regularity. 



When exposed to view under these conditions, the movements of the 

 mammalian heart are at once seen to be complicated to such a degree 

 that close examination is requisite to distinguish their different elements. 

 The most obvious appearance at first presented is the rapid succession 

 of two alternating conditions, namely a condition of rest and a condi- 

 tion of movement. Furthermore, if the heart be touched or gently 

 grasped between the fingers, it becomes evident that the two states of 

 rest and movement are accompanied by corresponding changes in the 

 consistency of the organ. At the time of rest it is comparatively soft 

 and yielding to the touch ; at the time of its movement, it becomes x hard 

 and tense. Inspection alone cannot determine which of these two states 

 corresponds with the entrance of the blood into the ventricles and which 

 with its exit ; in other words, which represents muscular relaxation and 

 which the contraction of the heart. Different observers, while watching 

 the movements of the same heart in the living animal, will often be led 

 to opposite conclusions in this respect. The only method of directly 

 determining the point is that first adopted by Harvey, in his observa- 

 tions upon the heart, which formed the basis of the discovery of the 

 circulation of the blood. If we insert through the walls of the left 

 ventricle a silver canula from one to two millimetres in diameter, so as 

 to pierce its cavity, the blood is forcibly projected from its orifice at the 

 time of the tension of the cardiac walls, while its flow is suspended in 

 the intervals of repose. 



Thus the two states of relaxation and tension of the heart correspond 

 with the relaxation and contraction of its muscular fibres. Like mus- 

 cular tissue elsewhere, that of the heart during relaxation is compara- 

 tively soft to the touch ; when the ventricles contract upon their contents 

 and forcibly expel the blood, they become tense and firm, by the sudden 

 rigidity of their fibres. By this means the two opposite conditions of 

 the diastole and systole of the ventricles may be recognized with cer- 

 tainty, and connected with the other corresponding phenomena of the 



1 Flint, Treatise on Diseases of the Heart. Philadelphia, 1870, pp. 61-62. 



