THE HUMAN STERNUM 49 



III. THE COMPARATIVE ANATOMY AND MORPHOLOGY 



OF THE STERNUM 



A study of the characters of the sternum and associated parts 

 In vertebrate animals lends support to the notion that the bone is axial in 

 position, and primarily connected with the shoulder-girdle, and secondarily 

 with the ribs ; that the human sternum is more primitive in type than the 

 quadrupedal sternum usually regarded as typical of the mammalia ; and 

 that the so-caUed segmentation of the sternum is a specialization associated 

 with the adoption of the quadrupedal position and the needs of locomotion. 

 Too much stress has in my opinion been laid by many anatomists upon the 

 similarities in structure of the fore and hind limbs. Even among quadrupeds 

 there are as many essential differences between them as likenesses, both in 

 structure and functions. And the sternum is In my view fundamentally 

 associated with the fore limbs. 



In the following pages a summary only is necessary of the comparative 

 anatomy of the sternum. The elaborate memoir of Parker" may be con- 

 sulted for the requisite detail. 



A. Fishes 



The sternum is absent among fishes as a rule. Instead, the elements 

 of the shoulder-girdle meet on the ventral surface of the body and form a 

 shield for the protection of the heart and branchial chamber. Fishes are 

 characterized by the great variety In the arrangements of the parts of the 

 shoulder-girdle. The simple type is found among cartilaginous fishes, such 

 as Acanthias vulgaris, in which the lateral halves of the shoulder-girdle are 

 confluent, and a continuous cartilaginous bar extends across the middle 

 line. Among chondrostean fishes, such as Adpenssr sturio, the coracoid 

 elements fail to meet, and clavicles and Interclavicle are interposed between 

 them. In Protopterus annectens there is present a median element, separate 

 from the coracolds and receiving the ends of the clavicles. In bony fishes 

 clavicles are present, and there Is a variable development of the coracoid 

 element. 



H 



