. A text-book of physiology : for medical students and physicians . ity is only potential,—that is, the parietal andvisceral layers are everywhere in contact with each other. Underpathological or accidental conditions air or exudations may enterthis space and form an actual cavity. Along the mid-line of thebody and around the roots of the lungs we have the mediastinalspaces lying between the pleural sacs of the two sides, but entirelyfilled with the various thoracic viscera, such as the heart, aorta andits branches, pulmonary artery and veins, venae cavse, azygos vein,trachea, esophagus, thora
. A text-book of physiology : for medical students and physicians . ity is only potential,—that is, the parietal andvisceral layers are everywhere in contact with each other. Underpathological or accidental conditions air or exudations may enterthis space and form an actual cavity. Along the mid-line of thebody and around the roots of the lungs we have the mediastinalspaces lying between the pleural sacs of the two sides, but entirelyfilled with the various thoracic viscera, such as the heart, aorta andits branches, pulmonary artery and veins, venae cavse, azygos vein,trachea, esophagus, thoracic duct, various nerves, and lymphglands. All these organs, therefore, lie outside the lungs. Aschematic view of these relations is represented in Fig. 263. The Thorax as a Closed Cavity.—The thorax is a cavity entirelyshut off from the outside and from the abdominal cavity. In thiscavity lie the lungs and the various viscera enumerated lungs may be considered as two large, membranous sacs, as 644 EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 645. represented in Fig. 263, the interior of which communicates freelywith the outside air through the trachea, glottis, etc., while theoutside of the sacs is protected from atmospheric pressure by thewalls of the chest. It is to be remembered, of course, that theinterior surface of the lungs is multiplied greatly by the sub-division into alveoli. It isestimated that the entireinner surface of the lungsamounts to as much as 90square meters, over one hun-dred times the skin surfaceof the body. The atmos-pheric pressure on the interiorsurfaces of the lungs expandsthese structures under normalconditions until they fill theentire thoracic cavity notoccupied by other the size of the chestcavity varies, that of thelungs must change accord-ingly; so that at all times thelungs fully fill up every partof the cavity not otherwiseoccupied. If the wall of thethorax is opened at anypoint so as to make commu-nication wit
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