[Illustration: _Views of the Diaphragm in the different states of Respiration._ Fig. CXLIV. Fig. CXLV. Fig. 144.—1. Diaphragm in its state of greatest descent in inspiration. 2. Muscles of the abdomen, showing the extent of their protrusion in the action of inspiration. Fig. 145.—1. Diaphragm in the state of its greatest ascent in expiration. 2. Muscles of the abdomen in action forcing the viscera and diaphragm upwards.] 387. By the action of the intercostal muscles, then, the capacity of the thorax is enlarged at the sides and from behind forward, or in its short diameter; by the action of the diaphragm, the capacity of the thorax is enlarged from above downwards, or in its long diameter; by the combined action of both, the capacity of the thorax is enlarged in every direction, and thus the motion of inspiration is completed. 388. Expiration, the respiratory motion which alternates with that of inspiration, consists of the diminution of the capacity of the thorax, which is effected by the converse motions of the same organs; that is, by the descent of the ribs and the ascent of the diaphragm. 389. By the descent of the ribs, the capacity of the thorax is diminished in its short diameter, because by this motion, the oblique arches of the ribs are approximated to each other and to the spinal column, and the sternum is also approximated to the spinal column. The descent of the ribs is effected first by the elasticity of their cartilages (fig. CXLI. 2). When the intercostal muscles relax, the force which raised the ribs ceases to be applied, and that moment the elasticity of the cartilages comes into play, and carries the ribs down wards. Secondly, by the contraction of the abdominal muscles (figs. CXLV. 2, and CXLVI. 6, 7, 8), the direct effect of which is to pull the ribs downwards (fig. CXLVI. 6, 7, 8). 390. By the ascent of the diaphragm the capacity of the thorax is diminished in its long diameter (fig. CXLV. 1). When the diaphragm ascends, it changes from the figure of an oblique plane (fig. CXLIV. 1), re-assumes its arched form (fig. CXLV. 1), and reaches as high as the fourth rib (fig. CXLV. 1). At the same time the abdominal muscles contract (fig. CXLV. 2), and are carried inwards towards the spinal column (fig. CXLV. 2). The result of these movements is, that the capacity of the thorax is diminished by all the space that intervenes between the lowest point of the oblique plane formed by the diaphragm and the fourth rib (fig. CXLV. 1), and by all the abdominal space lost by the contraction of the muscles of the abdomen (fig. CXLV. 2). [Illustration: Fig. CXLVI.—_View of the principal external Muscles of Respiration._ 1. The muscle called the Scalenus. 2. The muscles called the Intercostals. 3. Subclavius. 4. The bone called the Clavicle. 5. The muscle called the Serratus Magnus Anticus. 6. Obliquius Externus. 7. Rectus. 8. Obliquius Internus.]