ARTICULATIONS OF THE THORAX. ■ 193 



the asternal cartilages are bound together by a small yellow elastic ligament, which 

 is carried from the free extremity of each to the posterior border of the preceding 

 cartilage ; the anterior border of the first asternal cartilage is directly united to 

 the posterior border of the last sternal cartilage, through the medium of the 

 perichondrium and very short Kgamentous bands. This same asternal cartilage 

 is also bound to the inferior face of the xiphoid appendage by a small white 

 ligament (the chondro-xiphoicl), under which passes the anterior abdominal 

 artery. 



D. Sternal Articulation peculiar to the Ox and Pig. 

 It has been already shown that in these animals the anterior piece of the sternum is not 

 consolidated with the second portion. Tlie two are united by a diurthrodial articulation ; and 

 for this purpose the anterior presents a concave surface, the posterior a convex one. Bundles 

 of peripheral fibres firmly bind tliem to each other, and a special small synovial capsule facili- 

 tates their movements, which are very limited. 



The Articulations of the Thorax considered in a General Manner, 

 with Regard to Movements. 



The thorax can increase or diminish in diameter, in an antero-posterior and 

 a transverse direction ; whence arise the dilatation and contraction of this cavity — 

 the inspiratory movements accompanying the entrance of the external air into 

 the lungs, and the expiratory movements expelling the air contained in these 

 organs. 



The variations in the antero-posterior diameter of the chest being due to 

 changes in the figure of the diaphragm, need not be noticed here. But the 

 transverse variations being the result of the play of the costal arches on tjie spine 

 and sternum, it is advantageous to study the mechanism which presides in the 

 execution of theii' movements. 



The costal arches, being inclined backwards on the middle plane, the space 

 they enclose in their concavity is not nearly so extensive as if they were perpen- 

 dicular to this plane. Owing to their double arthrodial joints, the ribs are 

 movable on the spine ; and their inferior extremity, also movable, rests either 

 directly or indirectly on the sternum. Therefore it is that, when they are drawn 

 forward by their middle portions, they pivot on theii' extremities, and tend to 

 assume a perpendicular direction, which is the most favourable for the largest 

 increase of the space they limit ; then there is enlargement of the lateral diameter 

 of the thorax, which signifies dilatation of its cavity. The inverse movement, by 

 an opposite mechanism, causes contraction of the chest. 



The ribs are said to be elevated during the forward movement, and depressed 

 when they fall backwards. These expressions, though perfectly applicable to Man, 

 who stands in a vertical position, are not correct when employed in veterinary 

 anatomy. 



Besides the enlargement of the thorax in the transverse and the antero-posterior 

 directions, it is necessary to remark on an increase in a vertical direction, caused 

 by the displacement of the sternum forward, due to elevation of the ribs. In this 

 movement, the costal arches are erected not only on the median plane, but also 

 on the spine. The inferior extremity, carried forward, also takes the sternum 

 with it ; and this movement cannot take place without that piece being farther 

 removed from the vertebrae above. In Man, the displacement of the sternum is 

 very marked. 



