EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 637 



bronchioles which still possess in their walls cartilaginous plates and mus- 

 cular tissue and are lined by ciliated epithelial cells. The bronchioles give 

 off smaller branches whose walls show saccular dilatations similar to the alveoli 

 or air-sacs in which the respiratory exchange takes place. These bronchioles are 

 designated therefore as the bronchioli respiratorii (respiratory bronchioles). 

 They contain muscular tissue but no cartilage and their epithelium loses its cili- 

 ated character, taking on a low cuboidal form. The respiratory bronchioles di- 

 vide, giving off the alveolar ducts which form the last subdivision of the bronchial 

 tree. The alveolar ducts have air cells or alveoli in their walls and possess still 

 some muscular tissue. The distal end of each duct opens out into a variable num- 

 ber of irregularly spherical cavities, the atria, and each atrium in turn opens 

 into a number of irregular chambers, the alveolar air-sacs {sacculi alveolares). 

 The walls of these sacs are pouched to form a number of alveoli. Each alve- 

 olar duct with its dependent atria and air-sacs constitutes a primary lobule, 

 which forms the unit of structure of the lungs, as also its physiological unit. 

 The respiratory exchange between the blood and the alveolar air takes place 

 in the alveoli found in the walls of the respiratory bronchioles, the alveolar 

 ducts, the atria, and the ultimate sacculi. In the walls of these alveoli the 

 pulmonary capillaries are distributed. Miller calls attention to the fact that 

 the smooth muscle in the alveolar ducts shows a sphincter arrangement 

 round the openings into the saccuh, having a position, therefore, in which it 

 may exert an important control over the entrance of air into the ultimate 

 air-sacs. 



The great efficiency of this apparatus is evident when one re- 

 calls that every one of the infinite number of red corpuscles is 

 exposed separately to the air of the air cells, so that although the 

 time of transit is brief the entire amount of hemoglobin is nearly 

 completely saturated with oxygen. Each lung is enveloped in its 

 own pleural sac. The space between the parietal and the visceral 

 layer of each sac is the so-called pleural cavity, but it must be 

 borne in mind that under all normal conditions this cavity is only 

 potential, that is, the parietal and visceral layers are everywhere 

 in contact with each other. Under pathological or accidental con- 

 ditions air or exudations may enter this space and form an actual 

 cavity. Along the mid-line of the body and around the roots of 

 the lungs we have the mediastinal spaces lying between the 

 pleural sacs of the two sides, but entirely filled with the various 

 thoracic viscera, such as the heart, aorta and its branches, pul- 

 monary artery and veins, venae cavae, azygos vein, trachea, eospha- 

 gus, thoracic duct, various nerves, and lymph glands. All these 

 organs, therefore, lie outside the lungs. A schematic view of these 

 relations is represented in Fig. 263. 



The Thorax as a Closed Cavity. — The thorax is a cavity entirely 

 shut off from the outside and from the abdominal cavity. In this 

 cavity lie the lungs and the various viscera enumerated above. 

 The lungs may be considered as two large, membranous sacs, as 

 represented in Fig. 263, the interior of which communicates freely 

 with the outside air through the trachea, glottis, etc., while the 

 outside of the sacs is protected from atmospheric pressure by the 



