STRUCTURE AND PROPERTIES OF THE BLOOD-VESSELS. I2Q 



The conditions in the arterial system are the same as those just described. 

 The blood in the arteries is already engaged in continuous motion from the root 

 of the aorta to the capillaries (continuous movement) ; and the injection at inter- 

 vals of a mass of blood into the root of the aorta with each systole of the left 

 ventricle produces a positive wave (pulse) , which propagates itself with great 

 rapidity to the end of the arterial system, while the constant movement progresses 

 much more slowly. 



It is of great importance to compare the movements of fluids in rigid tubes 

 with the movements of fluids in elastic tubes. When a certain quantity of fluid 

 is forced into a rigid tube under a certain pressure, an equal quantity of fluid 

 will at once escape from the end of the tube, unless such a result is prevented by 

 the development of special resistances. The conditions are, however, different in 

 the case of an elastic tube. Immediately after the injection of a definite quantity 

 only a relatively small quantity of fluid escapes at first, the escape of the re- 

 mainder taking place only after the injecting force has subsided. 



If equal quantities of fluid are injected at intervals into a rigid tube, a corre- 

 sponding amount escapes with each impulse and the discharge continues as long 

 as the impulse, and the pause between each two periods of escape is always equal 

 to the period between two impulses. In the case of elastic tubes the conditions 

 are different. As the escape of the fluid continues for some time after the cessation 

 of the impulse, it will always be possible to establish a continuous outflow through 

 elastic tubes by making the interval between two injections shorter than the 

 duration of the outflow that takes place after the impulse has been completed. 

 Thus, the periodic injection of fluid into a rigid tube produces an isochronous, 

 sharply limited outflow of fluid, which can become permanent only when fluid 

 enters the tube in a continuous stream. In the case of elastic tubes, on the other 

 hand, intermittent introduction of fluid produces under the same conditions a 

 continuous outflow with systolic reinforcement. 



Hamel's investigations have shown that elastic tubes permit the passage of 

 more fluid when they are supplied in a rhythmical pulsatory manner than when 

 the fluid enters in an uninterrupted stream under constant pressure. The advan- 

 tage of the rhythmical impulse for the propulsion of the circulating fluid, as com- 

 pared with a uniform pressure, appears to reside in the fact that the alternating 

 movement preserves the elasticity of the arterial walls. 



STRUCTURE AND PROPERTIES OF THE BLOOD-VESSELS. 



The large blood-vessels in the body are designed solely for the purpose of 

 acting as conducting canals for the mass of blood, while the thin-walled capillary 

 vessels effect the interchange of substances between the blood and the tissues and 

 in the opposite direction. 



The Arteries differ from the veins in the possession of thicker walls in con- 

 sequence of the considerable development of muscular and elastic elements, as 

 well of a greatly developed middle tunic, with a relatively thin adventitial 

 coat. The walls of the arteries consist of three coats (Fig. 39) : 



The intima is lined on its inner surface by a nucleated endothelium (a) 

 consisting of flat, irregular, oblong cells. External to the endothelium is a thin, 

 finely granular layer containing more or less distinct fibers and numerous 

 spindle-shaped or stellate protoplasmic cells embedded in a corresponding system 

 of plasma-canals. To the outer side of this is the inner elastic layer (b), which 

 in the smallest arteries is represented by a structureless or fibrous, elastic mem- 

 brane and in the medium-sized arteries by a fenestrated membrane; while in the 

 largest it assumes the appearance of a stratified, fibrous or fenestrated, elastic 

 membrane consisting of two or three layers and united by connective tissue. All of 

 the larger and medium-sized arteries contain longitudinal fibers situated between 

 two elastic plates. Acting together with the circular fibers they are capable 

 of narrowing the caliber of the vessel ; but they possess also the faculty of widening 

 the lumen and maintaining it at a uniform width. On the other hand, it is im- 

 probable that they are capable of independent action or that such independent 

 action is capable of dilating the vessel. 



The middle coat has for its most characteristic constituent unstriated muscle- 

 fibers (c). In the smallest arteries this appears to be composed of scattered, 

 transverse, smooth muscle-fibers occupying an intermediate position between the 

 intima and the adventitia. The connecting material consists of a finely granular 

 tissue traversed by a few delicate elastic fibers. Passing from the smallest to the 

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