284 



DESIGN IN NATURE 



respiratory muscles generally, and also to other muscles. It is dome-shaped with the apex of the dome directed 

 upwards, and is compressed in an antero-posterior or lateral direction (Plate Ixxvii., Fig. 6, see also Figs. 48 and 49). 

 The vertebral column and the ribs forming it are beautifully curved, and both are supphed with cartilages— those of 

 the back-bone being known as inter- vertebral discs, and those of the ribs as costal cartilages. The latter, for the 

 most part, are attached to the ends of the ribs and the sides of the sternum or breast-bone. They give additional 

 elasticity and resiliency to the ribs. The ribs, which are twelve in number on either side of the body, are spiral, most 

 gracefully curved, and so springy and strong that they are sometimes converted into bows for discharging arrows. 

 The roots of the ribs are united to the vertebral column by loose joints which admit of vertical, horizontal, obhque, 

 and rotatory movements, so that they can readily respond to every movement occurring in the respiratory muscles. 



Fig. 48. 



Fig. 49. 



Fig. 48. — Anterior view of the skeleton of the thorax and abdomen of the human subject, u, a', Sternum or breast-l^one ; 

 b, h', vertebral coliinin or back-bone ; c, c', costal cartilages. 



Numbers 1 to 12 indicate the positions and attachments of the twelve ribs (after Holden). 



Fig. 49.. — Lateral view of the skeleton of the thorax and abdomen of the human subject, a, b, Costal cartilages ; c, c', sternum ; 

 fif, d', verteljral column. 



Numbers 1 to 12 indicate the positions and attachments of the twelve ribs (after Holden). 



The ribs are directed obliquely downwards and forwards, and are in the best possible position for being elevated and 

 carried outwards in inspiration, and depressed and carried inwards in expiration by the alternate closing or con- 

 traction and opening or relaxing movements of the thoracic and abdominal muscles (Plate Ixxviii. Figs. 1, 2, 3, and 4). 



Having exposed the cavities of the human chest and abdomen, and given frontal and side views of those 

 portions of the skeleton connected with respiration, we are now in a position to deal with .the respiratory muscles 

 themselves. 



The diaphragm, as being the chief muscle of respiration, falls first to be considered (Figs. 45 and 46, p. 281). The 

 diaphragm, next to the heart, is the most important muscle in the body. Its muscular fibres run in every direction, 

 in straight, curved, obhque, and transverse Hnes, in which respect it resembles the heart and hollow viscera generally. 

 It is endowed with universality of motion ; its chief movements being in a vertical direction — that is, from above 

 downwards and from below upwards. If the movements of the diaphragm become irregular, the breathing is at 

 once impaired, and if, by any chance, the muscle becomes paralysed, death inevitably results. Its highly complex 



