Osseous System. 159 



In some of the lower vertebrates parts remain cartilaginous which are 

 ossified in the higher. In some of the teleostean fishes, as well as the 

 cartilaginous, some bones of the skull are not ossified but remain carti- 

 laginous. This is also true of the frogs, the bones of the head being 

 only partly ossified, even in the adult. But the bones of the limbs of 

 the frog indicate the activity of the animal especially the hind limbs, 

 in which the tibia and fibula are fused together into one, as in the 

 horses, while the fore limbs are braced apart by very strong coracoids. 

 The backbone of the Ganoid fishes remains always cartilaginous, while 

 some of the ribs may become ossified. 



The position of bones in the body of the vertebrate must therefore 

 indicate the points where, in the working of the machine, the compression 

 strains finally bring up. The comparative anatomy of the shoulder and 

 pelvis bones, which furnish support to the limbs, illustrate this. 



The pectoral arch consists in man of the scapula or shoulder blade, 

 the clavicle or collar bone, and the sternum or breast bone the first two 

 paired, the last one single, five in all. The clavicle is attached at one 

 end to the outer angle of the spine of the scapula to what is called its 

 acromion process, at the tip and rear of the shoulder, and extends from 

 there to the breast bone with which it articulates. The effect of these 

 clavicles is to hold the shoulders apart and the longer they are the 

 greater is the breadth across the shoulders. They resist the compres- 

 sion strains that are generated by most of the movements of the arms. 

 The heaviest work of civilized man, which is lifting, generally tends to 

 pull the shoulders together, thus throwing a compression strain on the 

 clavicles. Swinging by the hands, as is the practice of the ape and 

 monkey tribes, and such movements as swimming, pushing sideways, 

 and, in short, nearly all our movements involve a strain of compression 

 often alternating with one of tension, upon the members of the pectoral 

 arch. There is a stout ridge of bone running diagonally across the 

 back of the shoulder blade to which large muscles are attached. This is 

 called the spine of the scapula, and the outer and front end of it forms 

 the acromion process mentioned above. 



Just inside of and to the front of this is another "process " of the 

 shoulder blade, projecting forward, which is called the coracoid process. 

 It serves in man, apes, &c. , as a place for the attachment of some mus- 

 cles and ligaments. 



But this is a piece of bone with a history. ' On comparison with other 

 animals it is found that this coracoid process is a remnant of a bone 

 which, in some of our vertebrate cousins, extends from the scapula to the 

 sternum nearly parallel with the clavicle, and serving the same general 

 purpose, except that the strain it sustains is further down the chest, as 

 its articulations at the ends are both lower than those of the clavicle. 



