ABDOMEN (in human anatomy.) In ex amining the human skeleton, we notice that i from the apex of the thorax to the inferior out let of the pelvis, there exists one great oblong excavation. The two superior fifths of this cavity are separated from the remaining portion in the entire subject by a musculo- tendinous lamella, which, thrown into a vaulted form, constitutes the partition between the cavity of the thorax above and that of the abdomen below. This latter cavity communicates inferiorly with the space circumscribed by the ossa innominata, denominated the cavity of the pelvis; nor is there any natural line of demarcation between the two cavities. The communication between the two cavities is as free in the recent subject as it is in the skeleton, and under various con ditions the contents of those cavities pass from the one to the other. A plane extended hori zontally from the linea iliopectinea on one side to the corresponding line on the other would constitute an artificial floor to the cavity of the abdomen, properly so called, and a limit be tween it and the pelvis; and this artificial divi sion of a cavity, naturally single, may be useful in describing the positions of viscera, but to understand the functions of the abdomen, it will be expedient to consider that cavity and the pelvis as one. Some anatomists ob ject to the use of the term cavity as applied to the abdomen, because no cavity can be said to exist, except in the skeleton or in the evisce rated subject ; neither can there properly be said to be a cavity of the thorax or of the cra nium, inasmuch as that cavity is obliterated so long as the viscera are in a state of integrity. I apprehend that the objection is hypercriti cal, as it must be evident that the cavity does not become apparent till the viscera have been removed ; nevertheless, it is perfectly correct to say that it contains the viscera, nor is it in correct to make use of the expression " anatomy of the abdominal cavity," to imply the anatomy of its contents when in their natural position.
Hence, then, we derive a natural subdivision, in treating the subject of this article, into two heads : 1. the anatomy of the walls of the abdomen ; and, 2. the anatomy of the cavity of the abdomen.
I. Of the walls of the abdomen.—One of the most striking differences between the abdomen and the other great visceral cavities consists in the small proportion of bone that exists in its walls. The osseous boundaries of the abdomen may be thus enumerated : superiorly, towards the posterior and outer part, the false ribs ; posteriorly, the lumbar region of the spine, which by its transverse processes affords strong points for the attachment of muscles, and by the bodies projects into the cavity, forming an imperfect septum, slightly convex on its anterior surface, and dividing the cavity into two symmetrical portions. Inferiorly, the alm of the ilia afford lateral expansions, which support some of the contents of the abdo men, and the pelvic brim completed behind by the promontory of the sacrum, forms the opening by which the cavity of the true pelvis communicates with that of the abdomen Between the inferior margin of the thorax and the superior margin of the false pelvis are stretched muscular lamellae and tendinous ap oneuroses, the cingulum abdominis musculoso aponeuroticum of Albinus and Haller, which, with integument, cellular membrane, &c. form the anterior, lateral, and for the most part the posterior walls of the abdomen, and circum scribe that space to which we have already alluded under the name of the cavity of the abdomen.
The superior wall of the abdomen is the diaphragm, and the inferior' wall of the abdo men, strictly so called, is formed by the ilia and their muscles, and is open in the centre at the superior outlet of the pelvis ; but if the abdominal and pelvic cavities be considered as one, then those parts which fill up the inferior outlet of the latter must be considered as likewise constituting the inferior wall of the former.