THORACIC CAVITY 141 



costal veins have already been referred to (p. 15). They 

 draw blood from the anterior part of the thoracic wall by 

 veins which accompany the intercostal branches of the 

 internal mammary arteries, and they terminate in the internal 

 mammary veins. 



The veins of the thoracic parietes are extremely variable, and the 

 description given above must be looked upon as representing merely their 

 more usual arrangement. 



THORACIC JOINTS. 



The dissector should now complete the dissection of the 

 thorax by an examination of the various thoracic joints. 



Dissection. When the portion of the sternum, with the 

 cartilages of the ribs, which was laid aside, is studied, the 

 following joints will be noted : inter-sternal, sterno-costal, and 

 inter-chondral. Very little dissection is necessary. After the 

 ligaments have been defined, the dissector should remove a thin 

 slice from the anterior aspect of each articulation, in order that 

 the interior of the joint may be displayed. 



Synchondrosis Sternalis. The joint between the manu- 

 brium and the body of the sternum is a Synchondrosis. The 

 opposing surfaces of bone are united by an intervening 

 plate of cartilage. The joint is supported by some anterior 

 and posterior longitudinal fibres which are developed in 

 connection with the strong and thick periosteum. The 

 posterior ligament is the stronger of the two. The joint 

 between the body of the sternum and the xiphoid process 

 is also a Synchondrosis till middle life, at which period the 

 two parts become ossified together. 



Articulationes Sternocostales. Seven ribs articulate with 

 each side of the sternum by means of their cartilages. 



The articulations of the first and the sixth are peculiar, 

 inasmuch as they articulate with single pieces of the sternum, 

 viz., with the manubrium and the lowest piece of the body, 

 respectively ; whereas each of the cartilages of the other true 

 ribs articulates with two segments of the sternum. The 

 cartilage of the first rib is implanted upon the side of the 

 manubrium ; there is no synovial cavity and the joint is a 

 Synchondrosis. The second costal cartilage is usually separated 

 from the sternum by two synovial cavities, between which an 

 interarticular ligament is developed. In the case of the 

 other joints it is more common to find a single synovial cavity 



