CIRCULATING APPARATUS IN MAN. 
223 
these, being attached to the neighbouring parts, serve to 
suspend the heart, as it were, in a cavity in which its 
movements may take place freely. This cavity is lined by 
a smooth serous membrane (§ 43), which, near its top, is 
vj ac t ac vj 
ar vc vr a vl 
Fig. 122.— Lungs, Heart, and principal vessels op Man. 
ar, right auricle; vr, right ventricle; vl, left ventricle; a, aorta; vc, vena cava; 
ac, carotid arteries; vj, jugular veins; as, subclavian artery; vs, subclavian veins ; 
t, trachea. 
reflected downwards over the vessels, and covers the whole 
outer surface of the heart. Hence as the surface of the heart, 
and the lining of the cavity in which it works, are alike 
smooth, and are kept moist (in health) with a fluid secreted 
for the purpose, there is as little interruption as possible from 
friction in the working of this important machine. 
257. The heart may be described as a hollow muscle, 
which, in Birds and Mammalia, as in Man, is divided into 
four distinct chambers. This division is effected by a strong 
vertical partition, that divides the entire heart into two halves, 
which are almost exactly similar to each other, excepting in 
the greater thickness of the walls on the left side ; and each 
of these halves (which do not communicate with one another) 
is again subdivided by a transverse partition, into two cavities, 
