PART III 

 THE CIRCULATION OF THE BLOOD 



SECTION VII 

 THE MECHANICS OF THE HEART 



CHAPTER XXIII 

 A COMPARATIVE STUDY OF THE CIRCULATORY SYSTEM 



General Arrangement of the Vascular System. — In its simplest 

 form the circulatory system consists of two parts, namely, a fluid 

 and a circular tube, the caliber of which is greatly increased at one 

 point to represent the pumping mechanism, or heart. The latter 

 first appears in the form of a simple bulbular enlargement of the gen- 

 eral vascular channel and finds its origin in the deposition of large 

 numbers of muscle cells possessing automatic properties. This enables 

 the walls of this organ to contract at intervals and to place the fluid 

 within under a higher pressure than that prevailing in the tubes with- 

 out. In consequence of this difference in pressure, the fluid is forced 

 through orifices (A) and (B) into the distal channel (C), but as every 

 phase of contraction of the musculature must necessarily be followed 

 by a phase of relaxation, the fall in pressure then resulting within the 

 heart must permit the fluid to return into the central cavity (H). 



A simple arrangement of this kind, however, is not adapted for 

 anything dynamically more perfect than an oscillatory to and fro 

 motion of the fluid. A true circular motion can only be imparted to 

 the fluid within this system by guarding the aforesaid orifices {A and 

 B) of the heart (H) by valves which open only in the direction of the 

 flow. These valves having been put in their proper, places, the con- 

 traction of the cardiac musculature now forces the fluid across the 

 yielding valve flap (A) into the distal channel (C), but is unable to drive 

 it through the opposite orifice (5), because this valve closes immediately 

 upon the first increase in the central pressure. A moment thereafter, 

 however, when the relaxation of the cardiac musculature has led to 

 the establishment of a lower central pressure, the valve at (B) is opened, 

 allowing the fluid to reenter the central compartment. Inasmuch 



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