KESPIRATION. 469 



external intercostal muscles proceed from the posterior border 

 of one rib, downwards and backwards to the anterior border 

 of that immediately behind. When, accordingly, the first rib 

 is fixed by the scalenus, these act powerfully in dilating the 

 whole cavity of the thorax. The levatores costarum are a 

 series of small muscles running from each dorsal vertebra 

 downwards and backwards, to the anterior border of the suc- 

 ceeding rib. Like those already noticed, they actively rotate 

 the ribs, turning their median part outwards and forwards. 

 Other accessory inspiratory muscles may be mentioned, espe- 

 cially the anterior serratus of the back, and in the dog the 

 latissimus dorsi. The former, in the horse, extends from the 

 dorsal spines downwards and backwards, to be inserted on the 

 sixth and following ribs, as far as the thirteenth inclusive. 

 The latissimus dorsi, which is attached anterior ly on the 

 inner aspect of the arm bone, is fixed to the two last ribs in the 

 ox, and to all the ribs which it covers in the dog and pig, so 

 that it greatly assists in the lateral dilatation of the thorax. The 

 abdominal muscles and diaphragm fix, to some extent, the last 

 rib, so that, in place of the latter being much advanced, the in- 

 tercostal spaces are widened, and any great diminution of the 

 antero-posterior diameter of the thorax prevented. This en- 

 largement of the intercostal spaces increases from the first to 

 the eleventh, twelfth, thirteenth, or fourteenth, from which 

 backwards a marked diminution may be noticed. In ordinary 

 calm respiration, the forward motion of the last rib is scarcely 

 if at all detectable. 



The dilatation of the thorax, in an antero-posterior direc- 

 tion, is due to the contraction of the diaphragm, a large 

 musculo-tendinous partition separating the thoracic from the 

 abdominal cavity. The diaphragm is muscular round its cir- 

 cumference and tendinous in the centre, towards which the 

 muscular fibres converge. It is attached to the ensiform 



