OF THE THORAX. 



443 



below its origin ; thus, each of the lower ribs receives fibres from the transverse 

 processes of two vertebra. 



Nerves. The muscles of this group are supplied by the intercostal nerves. 



Actions. The Intercostals are the chief agents in the movement of the ribs 



STERNO-MASTOID. 



SCALENUS MEDIUS. 

 SCALENUS ANTICUS. 



FIG. 289. Posterior surface of sternum and costal cartilages, showing Triangularis sterni muscle. (From a 

 preparation in the Museum of the Royal College of Surgeons of England.) 



in ordinary respiration. When the first rib is elevated and fixed by the Scaleiii, the 

 External intercostals raise the other ribs, especially their fore part, and so increase 

 the capacity of the chest from before backward : at the same time they evert their 

 lower borders, and so enlarge the thoracic cavity transversely. The Internal 

 intercostals. at the side of the thorax, depress the ribs and invert their lower 

 borders, and so diminish the thoracic cavity ; but at the fore part of the chest these 

 muscles assist the External intercostals in raising the cartilages. 1 The Levatores 



1 The view of the action of the Intercostal muscles given in the text is that which is taught by 

 Hutchinson (Cyd. of Anat. and Phys., art. "Thorax"), and is usually adopted in our schools. It is, 

 however, much disputed. Hamberger believed that the External intercostals act as elevators of the 

 ribs, or muscles of inspiration, while the Internal act in expiration. Haller taught that both sets of 

 muscles act in common viz. as muscles of inspiration and this view is adopted by many of the best 

 anatomists of the Continent, and appears supported by many observations made on the human subject 

 under various conditions of disease, and on living animals after the muscles have been exposed under 

 chloroform. The reader may consult an interesting paper by Dr. Cleland in the Journal of Anat. and 

 Phys. No. II.. May, 1867, p." 209, "On the Hutchinsonian theory of the Action of the Intercostal 

 Muscles," who refers also to Henle, Luschka, Budge, and Baumier, Obsermtions on (he Action of the 

 Intfrco.<tal Mu^le*. Erlangen. 1860. (In NewSyd. Soc.'gYear-Bookfor 1861, p. 69.) Dr. W. W. Keen has 

 come to the conclusion, from experiments made upon a criminal executed by hanging, that the Exter- 



