INFLAMMATION OF AllTEEIES AND VEINS. 257 



Fig. 82. 



A 



measure fruitlessly expended in tlie production of heat, and thus 

 wasted so far as the propulsion of blood is concerned ; whereas 

 it would, under ordinary circumstances, have been stored up in 

 a potential form, as tension of the elastic walls of the vessels, 

 ready to be reconverted at any moment into active energy. 

 Rigidity of a vessel therefore, while consuming the propelling 

 force of the heart, indirectly diminishes the 

 velocity of the blood-current beyond the rigid 

 point. Contraction of the tube from swelling 

 of its w^alls produces the same sort of effect, 

 and that directly, by opposing inclined planes 

 to the current of the blood ; a variable propor- 

 tion of the velocity is thus (according to well- 

 known mechanical laws) partly converted into 

 pressure, partly wasted as impact. Beyond the 

 rigid and contracted part of the vessel there- 

 fore, the movement of the blood is slower than 

 it should be. This retardation of the current 

 manifests itself especiall}^ in the extreme parts 

 of the systemic circulation, in the toes, finger- 

 tips, and nose. It may even amount in these 

 parts to complete stagnation, and so lead to 

 what is known as "spontaneous gangrene" 

 (cf. § D) an accident to which old ]:)eople are 

 liable. The state of things on the proximal 

 side of the rigid and contracted part is the 

 opposite of that just described. AYe know that 

 the pressure of the blood at an}- point of the 

 vascular system is proportionate to the resist- 

 ance to be overcome. If the resistance is 

 augmented (as it is both by rigidity and by 

 contraction) the pressure increases proportion- Arteiia cruraUs 



ately in those parts of the vascular system w^hicli with its brandies. 



^ 1 . T ,/ , , ,. -r "^ P Middle coat cal- 



are behmd the obstruction. Increase oi pros- citied. Natural 



sure in the entire aortic system is therefore a size. 

 common result of atheromatous changes. This stands in a com- 

 plementary relation to the diminution of jjressure and velocity 

 beyond the contracted and rigid parts. It is self-evident how- 

 ever that such increased pressure cannot last long without giving 

 rise to farther consequences. Amoiig these may be noticed : 



17 



