SECTION V. 



PHYSIOLOGY OF THE ORGANS OF CIRCULA- 

 TION OF THE BLOOD AND LYMPH. 



The heart and the blood-vessels form a closed vascular system 

 containing a certain amount of blood. This blood is kept in endless 

 circulation mainly by the force of the muscular contractions of the 

 heart. But the bed through which it flows varies greatly in width 

 at different parts of the circuit, and the resistance offered to the 

 moving blood is very much greater in the capillaries than in the 

 large vessels. It follows from the irregularities in size of the chan- 

 nels through which it flows that the blood-stream is not uniform in 

 character throughout the entire circuit; indeed, just the opposite is 

 true. From point to point in the branching system of vessels the 

 blood varies in regard to its velocity, its head of pressure, etc. 

 These variations are connected in part with the fixed structure of the 

 system and in part are dependent upon the changing properties of 

 the living matter of which the system is composed. It is con- 

 venient to consider the subject under three general heads: (1) 

 The purely physical factors of the circulation, that is, the me- 

 chanics and hydrodynamics of the flow of a definite quantity of 

 blood through a set of fixed tubes of varying caliber under certain 

 fixed conditions. (2) The general physiology of the heart and the 

 blood-vessels, that is, mainly the special properties of the heart 

 muscle and the plain muscle of the blood-vessels. (3) The innerva- 

 tion of the heart and the blood-vessels, that is, the variations in 

 the circulation produced by the action of the nervous system. 



CHAPTER XXV. 

 THE VELOCITY AND PRESSURE OF THE BLOOD-FLOW. 



The Circulation as Seen Under the Microscope. It is a 



comparatively easy matter to arrange a thin membrane in a living 

 animal so that the flowing blood may be observed with the aid of a 

 microscope. For such a purpose one generally employs the web 

 between the toes of a frog, or better still the mesentery, lungs, or 



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