266 COMPARATIVE PHYSIOLOGY. 



of blood, entailing an extra effort if it is to be expelled in addi- 

 tion to the usual quantity, which again leads to hypertrophy ; 

 ■but this is often suceeded by dilatation of the chambers of the 

 heart one after the other, and a host of evils growing out of 

 this, largely dependent on imperfect venous circulation, and 

 increased venous pressure. And it may be here noticed that 

 arterial and venous pressures are, as a general rule, in inverse 

 proportion to each other. 



If the quantity of blood in the ventricle, in consequence of 

 regurgitation, should prove to be greater than it can lift (eject), 

 the heart ceases to beat in diastole; hence some of the sudden 

 deaths from disease of the aortic valves. 



As a result of fatty, or other forms of degeneration, the 

 heart may suddenly rupture under strains. 



Actual experiment on the arteries of animals recently dead, 

 including men, shows that the elasticity of the arteries of even 

 adult mammals is as perfect as that of the vessels of the child, 

 so that man ranks lower than other animals in this respect. 



After a certain period of life the loss of arterial elasticity is 

 considerable and progressive. The arteries may undergo a de- 

 generation from fatty changes or deposit of lime ; such vessels 

 are, of course, liable to rupture ; hence one of the modes of 

 death among old animals is from paralysis traceable to rupture 

 of vessels in the brain. 



These and other changes also cause the heart more work, 

 and may lead to hypertrophy. Even in young animals the 

 strain of a prolonged racing career may entaU hypertrophy or 

 some other form of heart-disease. 



We mention such facts as these to show the more clearly 

 how important is balance and the power of ready adaptation 

 in all parts of the circulation to the maintenance of a healthy 

 condition of body. 



The heart is itself nourished through the coronary arteries ; 

 so that morbid alterations in these vessels cause, if not sudden 

 and painful death, at least nutritive changes in the heart-sub- 

 stance, which may lead to a dramatic end or to a slow impair- 

 ment of cardiac power, etc. 



Personal Observation. — The circulation is one of those de- 

 partments of physiology in which the student may verify much 

 upon his own person. The cardiac impulse, the heart's sounds 

 (with a double stethoscope), the pulse — its nature and changes 

 with circumstances, the venous circulation, and many other 



