468 RESPIKATION. 



the ribs and intercostal muscles. The thorax is com- 

 pressed from side to side at its anterior part, but approaches 

 the circular form posteriorly. This depends on the variation 

 in the form of the ribs, the first pair being strong, nearly 

 straight, and firmly fixed at their extremities, and the second 

 pair slightly curved in an outward and backward direction, 

 which curvature goes on increasing to the last. A number 

 of the anterior ribs called also the true ribs are attached 

 directly on the sternum by cartilaginous prolongations, while 

 the remainder, or false ribs, have their cartilaginous pro- 

 longations simply attached to those of the ribs next in 

 front. The true ribs are eight in number in the horse and 

 ox, seven in the pig, and nine in the dog. The false ribs 

 number ten in the horse, five in the ox, seven in the 

 pig, and four in the dog. The false ribs have neces- 

 sarily a freer action than the others, a circumstance ac- 

 counting for the freedom of respiration in the horse; never- 

 theless, in certain animals, as in the dog, this is made up 

 by the supply of additional and powerful inspiratory 

 muscles. 



Inspiration is that process by which the cavity of the 

 thorax is enlarged, and the lungs inflated. The dilatation of 

 the thorax takes place in a lateral and an antero-posterior 

 diameter. 



Lateral dilatation is effected by a slight rotatory move- 

 ment of the ends of the ribs, the median part being at the 

 same time turned outwards and forwards. The above move- 

 ment is chiefly dependent on the action of the scaleni, exter- 

 nal intercostal and levatores costarum muscles. The scalenus 

 extends from the four last cervical vertebrae to the first ribs, 

 which it fixes, or even pulls slightly forwards. In the ox it 

 extends to the fourth, and in the dog as far as the sixth rib, 

 so that its direct action on these is much extended. The 



