THE HUMAN STERNUM 15 



have fused with it, the succeeding (fourth and fifth) ribs have as yet no 

 connexion with the sternal leiste. Yet the sternal strip is continuous, so 

 that either the fourth and fifth ribs have advanced and afterwards receded, 

 or their connexion with the sternum is a secondary one. 



3. At this admittedly early period there is no indication of segmenta- 

 tion of the sternum, and no proof is given of any growth of the rib ends to 

 produce it by their fusion. 



4. It is shown that the inner ends of the clavicles and the pre- 

 sternum are in intimate organic association at an early period : the parts of 

 the sterno-clavicular joint, the ligaments, menisci and suprasternal ossicles, 

 as well as the upper end of the presternum itself, being differentiated out of 

 a homogeneous material. 



5. The presternum Ruge derives from the cartilages of the first 

 and second ribs, but he cannot determine that the suprasternal ossicles have 

 a costal origin. 



6. With regard to the formation of the sternum and ribs, he states 

 twice that there is a difference in the microscopic structure of the two. 

 Once he states that the sternal leiste appear purely cellular, while (in 

 the same specimen) the ribs have a considerable amount of intercellular 

 tissue ; and in another place, speaking of the appearance of the elemental 

 structure of the cartilage, he states that the rib cartilage contains cells larger 

 than those of the sternum. 



7. The metasternum Ruge derives from the distal ends of the eighth 

 and ninth costal cartilages, which he supposes have lost a primitive connexion 

 with the sternum. Albeit, he describes a pre-existing tissue continuous with 

 the mesosternum, extending into the abdominal wall, and, when first seen, 

 unconnected with the ribs in question. 



There are other points of detail which, in addition to those named, 

 support my contention that Ruge's observations can be used to support a 

 thesis exactly opposite to the one advanced by him. 



From his observations one contends that a picture is presented of a 

 mesial presternum and metasternum, and a mesosternum continuous with 

 both, but thicker on either side than in the mesial plane. The presternum Is 

 continuous with the shoulder-girdle (coracoid and clavicle) ; the meta- 



