DIAPHRAGM (in anatomy), (1c4ecenta, a, inter, and rp ecca sepia, claudo ; Lat. din ph•ognu, ; Ital. digframma ; Fr. diaphragms; Ger. Zwerehiell ; Eng. the name given to that musculo-tcndinous septum by which the cavities of the thorax and abdomen are separated from each other in the Mammalia.
Nothing analogous to the diaphragm of mam mals can be detected in the Invertebrate classes of animals ; the function of which it is a princi pal museularagent in the Mammalia, respiration, being effected by the skin, intestines, stigmata, trachea, gills, &c. Most of the Vertebrata, however, exhibit something analogous to the diaphragm. Thus in Fishes the muscular sep tum dividing the cavity of the branchial ap paratus (thorax) from the abdomen bears a certain resemblance to the diaphragm. Birds have muscles which proceed obliquely upwards in the form of flat bundles of fibres from the middle of the lower ribs to the under part of the lungs, where they are lost in the pleura covering these organs ; and thus by their con traction depress the lungs themselves, expand their cells, and facilitate the ingress of air into them. These muscular fibres are particu larly developed in the parrot" But, as has been said, it is only in Mammalia that the genuine diaphragm is to be found ; and all the animals of this class possess it. The organ, as
might be expected, undergoes some modifica tions in different families. In amphibious and cetaccous mammalia it approximates to that of birds. A very strong and fleshy diaphragm is attached to the dorsal side of the cavity of the trunk so low down that it ascends considerably in order to be connected in a peculiar manner with the upper and anterior extremity of the abdominal muscles ; so that the lungs lie be hind rather than above the diaphragm.* In the porpoise there is no central tendon.t The horse, elephant, rhinoceros, and other animals whose ribs approach the pelvis, have a very extensive diaphragm, which forms an elevated arch towards the thorax./ This shape is neces sary to accommodate the bulky contents of the abdomen, without altering the attachments of the muscle, which, as in man, are connected to the lowest ribs. Some other variations from the structure and form of the diaphragm in man might be noticed, but they are very unim portant. NVe shall therefore proceed to give a detailed account of the muscle in the human subject.