ACTIONS OF RESPIRATORY MUSCLES. 325 



collapsed condition, not descending- so far as the attachments of the diaphragm. The central 

 lobe of the tendon is slightly depressed where it underlies the heart, being about the level of 

 the xiphi-stemal articulation. The vault of the diaphragm rises higher on the right than on 

 the left side. In the dead body it rises on the right side to the level of the junction of the 

 fifth rib-cartilage with the sternum, and on the left side only as high as the sixth : this 

 difference is connected with the great size and firmness of the liver on the right side. It is 

 covered superiorly by the pleurae and the pericardium, the fibrous layer of the latter mem- 

 brane blending with the tendinous centre, as well as with the fascia covering its muscular 

 substance. The lower surface, of a deeply concave form, is lined by the peritoneum, and has 

 in apposition with it the liver, the stomach, the pancreas and spleen, and the kidneys. 



Varieties. The sternal portion of the muscle is not unfrequently wanting. Asa very rare 

 occurrence a fleshy fasciculus has been seen passing from the upper surface of the diaphragm 

 to the wall of the oesophagus. 



Nerves. The intercostals, levatores costarum, subcostals, and triangularis stemi are 

 supplied by the intercostal nerves. The diaphragm is mainly supplied by the phrenic nerves 

 from the fourth and fifth cervical ; but it also receives small twigs from the lower intercostal 

 nerves (Luschka), and sympathetic filaments from the plexuses round the phrenic arteries. 



Actions. Movements of respiration. The mechanical act of respiration consists of two 

 sets of movements, viz., those of inspiration and of expiration, in which air is successively 

 drawn into the lungs and expelled from them by the alternate increase and diminution of the 

 thoracic cavity. The changes in the capacity of the thorax are effected by the expansion and 

 contraction of its lateral walls, called costal respiration, and by the depression and elevation 

 of the floor of the cavity, through contraction and relaxation of the diaphragm, called 

 diaphragmatic or abdominal respiration. These two movements are normally combined in the 

 act of respiration, but in different circumstances one of them is resorted to more than the 

 other. Thus, abdominal respiration predominates in the male, while costal respiration is 

 employed to a greater extent in the female. 



Inspiration. The study of the movements of the thoracic walls in respiration presents 

 considerable difficulty from the complexity of these movements, and from the impossibility of 

 perfectly imitating in the dead body the mechanical conditions under which they occur during 

 life. On a fresh ligamentous thorax, by raising and depressing the sternum, the ribs may be 

 moved upwards and downwards nearly parallel to one another ; the first rib moving as freely 

 as the others. But during life several causes combine to make the first rib more fixed than 

 those which follow ; as for example, the weight of the upper extremity, and the strain of the 

 intercostal muscles and ribs below. The movements of the thoracic walls in respiration are 

 as follows : 1st. The antero -posterior diameter is increased by a forward movement of the 

 sternum with the attached ribs and cartilages ; the lower end of the sternum being raised and 

 advanced, while the upper end, which in easy respiration is at rest, or nearly so, is only raised 

 in full inspiration. 2nd. The transverse diameter of the thorax is increased by the elevation 

 and the eversion of the ribs, the first of these movements bringing larger costal arches to a 

 level occupied in expiration by smaller arches above them, and the second increasing the 

 width of their arch outwards (v. p. 161). These movements are facilitated by the elasticity 

 of the ribs and, to a greater extent, of the cartilages, allowing of the opening out of the angle 

 between the two, while the capacity of the thorax is also increased in its inferior part by the 

 simultaneous backward movement of the lower ribs, due to the arrangement of the costo- 

 transverse articulations. 3rd. The vertical diameter of the thoracic cavity is increased by the 

 descent of the platform of the diaphragm forming its floor ; and as the lower ribs are drawn 

 backwards and outwards rather than raised, while the lasc rib may even move somewhat 

 downwards, the depth of the hinder part of the cavity is by this means also slightly 

 augmented. Lastly, it may be remarked, that extension of the vertebral column is an 

 important agent in respiration, for when the column is bent forwards, the ribs are pressed 

 together in the concavity of the curve, and, conversely, when the column is extended, the ribs 

 are separated. 



Action of the 'intewoxtttl nmxrlrx. The manner in which these muscles act has been a 

 subject of controversy from an early time, and is not yeb thoroughly determined. It is now 

 generally agreed that the external muscles are elevators of the ribs, and therefore muscles of 

 inspiration, but as to the action of the internal muscles there is still considerable difference of 

 opinion. According to one view, defended by Haller, the external and internal layers have a 

 common action, the decussating fibres acting in the direction of the diagonal between them ; 

 while according to another view, that of Hamberger, the external intercostal muscles are 

 admitted to be elevators, but the internal are held to be depressors of the ribs. More recently 

 these views have been modified by Hutchinson to the extent of admitting that the external 

 intercostal muscles, and the parts of the internal intercostals placed between the costal 

 cartilages, elevate the ribs, and that the lateral portions of the internal intercostals act as 

 depressors. This view is illustrated mechanically, and supposed by some to be demonstrated, 

 by means of a mechanism of rods and elastic bands imitating the conditions of the ribs. But 



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