312 MUSCLES OF THE THOKAX. 



consists of two sets of movements, viz., those of inspiration and of expii'ation, in 

 which ail- is successively di-awn 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 deijression and elevation of the floor of 

 the cavity, through contraction and relaxation of the diaphi-agm, called ahdominal 

 rcsinration. These two movements are normally combined in the act of respira- 

 tion, but in different circumstances one of them is resorted to more than another. 

 Thus, abdominal respiration is most employed in the male, costal resjpiration most 

 in the female. 



Inspiration. — The study of the movements of the thoracic walls in resphation 

 presents considerable difficulty from the complexity of these movements, and 

 from the impossibility of perfectly imitating m the dead body the mechanical 

 conditions ruider which they occur in life. On the prepared skeleton, 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 

 dui'ing life several causes combine to make the fii'st 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 beneath. 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 is raised and advanced, while the upper end, which in 

 easy respu-ation is at rest, or nearly so, is only raised in full insi^iration. 2nd. 

 The lateral diameter of the thorax is increased by the elevation and the rotation 

 of the ribs ; the first of these movements bringing larger costal arches to a level 

 occupied in expu-ation by smaller arches above them ; and the second, by the 

 rotation of the ribs round an axis extending from their vertebral extremities 

 to the sternum, which everts the lower edgje of the ribs, and increases the width 

 of their arch outwards. 3rd. The capacity of the thorax, transversely and 

 antero-posterioiiy. is increased by the elastic bending of the ribs, as well as the 

 opening of the angle between the ribs and their cartilages, produced by the 

 resistance of the sternum and weight of the limbs to the forward and upward 

 motion of the extremities of the cartilages, and by the inclination backwards 

 given to the middle ribs in their ujiward movement by the oblique direction of 

 the plane of the costo-transverse articular surfaces. 4tli. Tlie vertical diameter 

 of the thoracic cavity is increased by the descent of the i^latfonn of the dia- 

 phragm fonning its floor ; but as any consideraljle elevation of the ribs woidd 

 tend to diminish the capacity of the thorax in inspiration, the lowest ribs 

 are drawn backwards and outwards rather than raised. Lastly, it may be re- 

 marked, that extension of the vertebral column is an important agent in respira- 

 tion, for when the column is bent forwards, the ribs are pressed together in the 

 concavity of the cmwe, and, conversely, when the colmnn is extended the ribs 

 are separated. 



Action of the Intercostal JL/scles. — The manner in which these muscles act has 

 been a subject of controversy from an early time, and is not yet thoroughly deter- 

 mined. Among those who look upon the intercostal muscles as active in respira- 

 tion, all are agreed that the external muscles are elevators of the ribs, and 

 therefore muscles of inspu'ation. 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 whole internal are held to be depressors of the ribs. More recently these 

 views have been modified hy 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 the ribs differ from 

 such rods in respect that they are not straight or rigid bars, and are not 

 free at either end, but are deeply cuiwed, and have the greatest extent of 



