through
its substance, in the manner of the nutrient vessels of all other organs.
The two coronary arteries of the heart arise from the systemic aorta
immediately outside the semilunar valves, situated in the root of this
vessel, and in passing right and left along the auriculo-ventricular
furrows, they send off some branches for the supply of the organ itself,
and others by which both vessels anastomose freely around its base and
apex. The vasa cordis form an anastomotic circulation altogether
isolated from the vessels of the other thoracic organs, and also from
those distributed to the thoracic parietes. The coronary arteries are
accompanied by veins which open by distinct orifices (foramina Thebesii)
into the right auricle. Like the heart itself, its main vessels do not
depend for their support upon the blood conveyed by them, but upon that
circulated by the small arteries (vasa vasorum) derived either from the
vessel upon which they are distributed, or from some others in the
neighbourhood. These little arteries are attended by veins of a
corresponding size (venules) which enter the venae comites, thus
carrying out the general order of vascular distribution to the minutest
particular. Besides the larger nerves which accompany the main vessels,
there are delicate filaments of the cerebro-spinal and sympathetic
system distributed to their coats, for the purpose, as it is supposed,
of governing their "contractile movements." The vasa vasorum form an
anastomosis as well upon the inner surface of the sheath as upon the
artery contained in this part; and hence in the operation for tying the
vessel, the rule should be to disturb its connexions as little as
possible, otherwise its vitality, which depends upon these minute
branches, will, by their rupture, be destroyed in the situation of the
ligature, where it is most needed.

XI.--The branches of the systemic aorta form frequent anastomoses with
each other in all parts of the body. This anastomosis occurs chiefly
amongst the branches of the main arteries proper to either side. Those
branches of the opposite vessels which join at the median line are
generally of very small size. There are but few instances in which a
large blood vessel crosses the central line from its own side to the
other. Anastomosis at the median line between opposite vessels happens
either by a fusion of their sides lying parallel, as for example (and
the only one) that of the two vertebral arteries on