DISEASES OF HEART, BLOOD VESSELS, AND LYMPHATICS. 249 
The action of the heart is governed and maintained by the pneumo- 
gastric nerve (tenth pair of cranial nerves); it is the inhibitory 
nerve of the heart, and regulates, slows, and governs its action. When 
the nerve is cut, the heartbeats increase rapidly, and, in fact, the 
organ works without control. When the nerveis iendaly uvitatad the 
holdback, or inhibitory force, is increased, and the heart slows up in 
the same measure. The left cavities of the heart, the pulmonary 
veins, and the aorta, or systemic artery, contain red or florid blood, 
fit to circulate through the body. The right cavities of the heart, 
with the venz cave, or systemic veins, the pulmonary artery, contain 
dark blood, which must be transmitted through the lungs for reno- 
vation. 
The arteries, commencing in two great trunks, the aorta and the 
pulmonary artery, undergo division, as in the branching of a tree. 
Their branches mostly come off at acute angles, and are commonly of 
uniform diameter in each case, but successively diminish after and in 
consequence of division, and in this manner gradually merge into the 
capillary system of blood vessels. As a general rule, the combined 
- area of the branches is greater than’ that of the vessels from which 
they emanate, and hence the collective capacity of the arterial system 
is greatest at the capillary vessels. The same rule applies to the 
veins. The effect of the division of the arteries is to make the blood 
move more slowly along their branches to the capillary vessels, and 
the effect of the union of the branches of the veins is to accelerate the 
speed of the blood as it returns from the capillary vessels to the 
venous trunks. 
In the smaller vessels a frequent running together, or anastomosis, 
occurs. This admits of a free communication between the currents of 
blood, and must tend to promote equability of distribution and of 
pressure, and to obviate the effects of local interruption. The arteries 
are highly elastic, being extensile and retractile both. in length and 
breadth. During life they are also contractile, being provided with 
muscular tissue. When cut across they present, although empty, an 
open orifice; the veins, on the other hand, collapse. 
In most parts of the body the arteries are inclosed in a sheath 
formed of connective tissue, but are connected so loosely that, when 
the vessel is cut across, its ends readily retract some distance within 
the sheath. Independently of this sheath, arteries are usually de- 
scribed as being formed of three coats, named, from the relative 
positions, external, middle, and internal. This applies to their struc- 
ture so far as it is discernible by the naked eye. The internal, serous, 
or tunica intima, is the thinnest, and is continuous with the lining 
membrane of the heart. It is made up of two layers—an inner, con- 
sisting of a layer of epithelial scales, and an outer, transparent, 
whitish, highly elastic, and perforated. The middle coat, tunica 
