on.

T. Convolutions of the small intestines distended with air.

[Illustration: Chest and abdomen, showing bones, blood vessels, muscles
and other internal organs.]
Plate 23

COMMENTARY ON PLATE 24.

THE RELATIONS OF THE PRINCIPAL BLOODVESSELS
TO THE VISCERA OF THE THORACICO-ABDOMINAL CAVITY.

The median line of the body is occupied by the centres of the four great
systems of organs which serve in the processes of circulation,
respiration, innervation, and nutrition. These organs being fashioned in
accordance with the law of symmetry, we find them arranged in close
connexion with the vertebrate centre of the osseous fabric, which is
itself symmetrical. In this symmetrical arrangement of the main organs
of the trunk of the body, a mechanical principle is prominently
apparent; for as the centre is the least moveable and most protected
region of the form, so have these vitally important structures the full
benefit of this situation. The aortal trunk, G, of the arterial system
is disposed along the median line, as well for its own safety as for the
fitting distribution of those branches which spring symmetrically from
either side of it to supply the lateral regions of the body.

The visceral system of bloodvessels is moulded upon the organs which
they supply. As the thoracic viscera differ in form and functional
character from those of the abdomen, so we find that the arterial
branches which are supplied by the aorta to each set, differ likewise in
some degree. In the accompanying figure, which represents the thoracic
and abdominal visceral branches of the aorta taken in their entirety,
this difference in their arrangement may be readily recognised. In the
thorax, compared with the abdomen, we find that not only do the aortic
branches differ in form according to the variety of those organs
contained in either region, but that they differ numerically according
to the number of organs situated in each. The main vessel itself,
however, is common to both regions. It is the one thoracico-abdominal
vessel, and this circumstance calls for the comparison, not only of the
several parts of the great vessel itself, but of all the branches which
spring from it, and of the various organs which lie in its vicinity in
the thorax and abdomen, and hence we are invited to the study of these
regions themselves connectedly.

In the thorax, the aorta, G G*, is wholly concealed by the lungs in
their states both of inspiration and