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Show 562 PROF. HUXLEY ON THE RESPIRATORY [June 20, the sternum. From the median line, the two halves of the oblique septum slope laterally and ventrally until they attach themselves to the parietes of the abdomen behind, to those of the thorax more anteriorly, and to the margins of the sternum in front, at a considerable distance from the attachment of the pulmonary aponeurosis. It follows that a wide interspace is left, on each side, between the pulmonary aponeurosis, dorsally and laterally, the dorsal median septum in the middle line, the oblique septum ventrally and below, and the parietes of the body laterally ; and, as the mesial attachments of the pulmonary aponeurosis and of the oblique septum are very much closer together than their outer attachments, the whole cavity has somewhat the form of a wedge with the narrow edge towards the middle line. This subpulmonary chamber is divided into four loculi by three dissepiments, which pass transversely from the lateral face of the oblique septum to the mesial face of the pulmonary aponeurosis. Each loculus lodges one of four postbronchial saccular diverticula of the wall of the lung, constituting the proper air-sacs, which thus fill up the subpulmonary chamber, between the insertion of the bronchus and its posterior extremity. Another air-sac lies in front of the insertion of the bronchus, on the mesial side of the anterior extremity of the lung. Thus, on each side, there are five air-sacs, the lateral and superior face of each of which abuts against a particular region of the lung. The most anterior is that just mentioned, which may be called pra-bronchial (" reservoir cervical," Sappey), as the bronchus lies between it and the next, or subbronchial ("interclavicular," Sappey). The dissepiment which separates the loculus of this sac from the next is attached mesially along the oblique elevation which runs from the insertion of the bronchus to the ventral margin of the lung. The following dissepiment is attached dorsally, near the origin of the cceliac artery from the aorta, which lies in the median dorsal septum, and thence to the pulmonary aponeurosis, along a line which is inclined more or less obliquely backwards, to the posterior ventral margin. It separates two loculi, which lodge the anterior and the posterior intermediate air-sacs (" reservoir diaphragmatique anterieur et posterieur," Sappey). The third dissepiment, still more inclined, divides the loculus of the posterior intermediate air-sac from that of the posterior air-sac ("reservoir abdominal," Sappey). Thus, that part of the thoraco-abdominal cavity which lies dorsad and anterior to the oblique septum lodges no other viscera than the lungs and the air-sacs, and may therefore be distinguished, as the inspiratory cavity, from the cardio-abdominal cavity, which contains the heart and the rest of the viscera, and lies below and behiud the oblique septum. The respiratory cavity is further divided into two lateral chambers by the median dorsal septum; and each of these chambers is subdivided by the pulmonary aponeurosis into two stories, of which the upper is occupied by the lung, and the lower by the loculi with their contained air-sacs. The dorsal aorta traverses the median dorsal septum from before backwards, giving off, from its ventral aspect, the cceliac and the |