DISSECTION OF THE BACK AND THORAX. 137 



Synovial Sac. — This will be exposed by removing the posterior costo- 

 transverse ligament. In the last two or three ribs there is no separate 

 synovial sac for the costo-transverse joint ; but the posterior costo- 

 central sac is extended over it, the two articular surfaces being in 

 these ribs continuous. 



Chondro-Costal Joint. The inferior extremity of the rib is slightly 

 excavated, and receives the extremity of the costal cartilage. The 

 periosteum passes from the rib to the cartilage, and serves to consolidate 

 the union. 



Chondro-Sternal Joint. This is the joint by which the costal 

 cartilage of each of the first eight ribs is articulated to the sternum. 

 Peripheral fibres envelop the joint and form a capsular ligament. 

 Above and below the joint the capsule is somewhat thickened, forming 

 the superior and inferior costo-sternal ligaments. 



Synovial Sac. — The joint possesses a synovial membrane. The 

 cartilages of the first pair of ribs meet in a common joint on the middle 

 line, and there is a single synovial sac common to these chondro-sternal 

 articulations and to the facet between the two cartilages. 



Union of the Costal Cartilages to one another. The cartilage 

 of the first asternal rib (9th) is firmly bound to the preceding carti- 

 lage by short fibrous bands. It is further bound to the lower face 

 of the xiphoid cartilage by a small band — the chondro-xiphoid ligament. 

 From the tip of each succeeding cartilage a yellow elastic band is 

 carried to the posterior edge of the cartilage in front of it. 



Movements of the Ribs. — Each rib with its cartilage moves round 

 an imaginary axis joining the head of the rib and the sternal end of 

 the cartilage. In inspiration the rib moves forwards and outwards 

 round this axis, so as to bring the middle portion of the rib towards the 

 position occupied by the preceding rib at the end of expiration. This 

 movement lengthens the line joining the mid point of each rib to the 

 corresponding point on the opposite rib, and thus increases the capacity 

 of the chest by increasing its transverse diameter. During expiration 

 the rib falls into its original position by moving in the opposite direction. 

 In these movements the head of the rib and the extremity of the costal 

 cartilage rotate slightly in their cavities, but without change of place. 

 The tubercle of the rib glides on the facet of the transverse process, 

 moving in a circle whose centre is the costo-central joint. 



The Sternum. There are no joints in the sternum of the horse, 

 in which the osseous segments are simply united by persisting portions 

 of the original cartilaginous mass. In this connection, however, there 

 may be noticed the two suprasternal fibrous cords which pass, one 

 at each side of the thoracic surface of the sternum, internal to the 

 mammary vessels. 



