THE CIRCULATION OF THE BLOOD. 283 
Asa 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 middle life the loss of arterial elasticity is consider- 
able and progressive. The arteries may undergo a degenera- 
tion from fatty changes or deposit of lime; such vessels are, of 
course, liable to rupture; hence one of the frequent modes of 
death among old persons 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 persons the 
strain of a prolonged athletic career may entail 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 depart- 
ments 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 cir- 
cumstances, the venous circulation, and many other subjects, 
are all easy of observation, and after a little practice without 
liability of causing those aberrations due to the attention being 
drawn to one’s self. 
The observations need not, of course, be confined to the stu- 
dent’s own person; it is, however, very important that the nor- 
mal should be known before the observer is introduced to cases 
of disease. Frequent comparison of the natural and the dis- 
eased condition renders physiology, pathology, and clinical 
medicine much good service. We again urge upon the student 
to try to form increasingly vivid and correct mental pictures 
of the circulation under its many changes. 
Comparative.—An interesting arrangement of blood-vessels, 
known as a rete mirabile, occurs in every main group of verte- 
