302 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



girdles is formed in cartilage, has been partly or entirely replaced 

 in all mammals by the development over it of the clavicle, a 

 dermal or membrane-formed bone, the first of all the bones 

 to ossify. There is thus no true representative of the clavicle 

 in the pelvis. The inter-clavicle so strongly developed in 

 the ornithorynchus and in the "merry-thought" of the fowl 

 is also a dermal bone. It is represented in man by the inter- 

 clavicular ligament. It is highly probable that the clavicle does 

 not completely replace the pre-coracoid. The epi-coracoid of 

 ornithorynchus probably represents part of this element (Fig. 

 244). It is also represented in the shoulder girdle of man by 

 three structures (see Fig. 245). (1) By the cartilaginous inner 

 end of the clavicle ; (2) by the inter-articular cartilage ; (3) by 

 the supra-sternal bones, all of which appear in cartilage. 



In order to give greater mobility and speed to some four-footed 

 mammals, the clavicle has been reduced to a ligamentous band, 

 except at its extremities (rabbit, dog, etc.). In climbing animals, 

 and those in which the power of grasping or embracing is highly 

 developed, the clavicles are fully developed. 



The acromion process is ossified from several centres which 

 appear in the years of adolescence ; the epiphysis so formed may 

 be united to the spine by fibrous tissue only. This occurs in 

 over 8% of subjects (Symington), and may be mistaken for a 

 fracture of the process. The coraco-clavicular ligaments may be 

 derived from the pre-coracoid element. 



The Hand and Foot. — The hand and foot of man, as is the 

 case in all primates, retain the primitive arrangement of elements 

 much more closely than do most other mammalian orders. The 

 primitive type of hand or foot, out of which the various forms 

 found in mammals have been modified, are seen in such reptiles 

 as the lizard or tortoise (Fig. 246). In the hand of man the 

 same bones are to be seen as in the tortoise (Fig. 246), and the 

 same arrangement with some exceptions. The elements in the foot 

 of a typical lizard resemble closely the arrangement seen in its 

 hand ; the same elements are present even in the highly modified 

 human foot. The hand and foot bones have undergone great 

 specialization in most mammals. In the evolution of the horse, 

 for instance, one lateral digit after another has become vestigial, 

 leaving the central digit enormously enlarged and specialized to 



