230 ELEMENTARY ANATOMY. [LESS. 



In the first place, it appears that the appendicular appa- 

 ratus is, in the developed skeleton, no mere portion of the 

 axial skeleton, but is (in all forms as yet known) a distinct 

 system of parts appended to and more or less closely and 

 variously connected with the axial system. 



1 8. The upper limbs in man are suspended, as we have 

 seen, from an incomplete bony girdle attached to the backbone 

 on its dorsal aspect, by soft parts only ; but on the ventral 

 aspect abutting against the median portion of the paraxial 

 system, i.e. against the sternum doing this, nevertheless, 



FIG. 198. Transverse Section of the Thorax of Man, showing the relation borne 

 by the appendicular skeleton (shoulder-girdle) to the axial skeleton : the latter 

 is represented by the black parts of the figure. 



n, neural canal ; s, neural spine ; tc, thoracic cavity ; s, sternum ; c, clavicle ; 

 sc, scapula (cut through) ; A, head of the humerus. 



only with the clavicles. We have seen, however, that man 

 has but an imperfect shoulder-girdle compared with what is 

 possible, for in some animals this part joins the sternum by 

 great coracoids, to which pre-coracoid bones are appended, as 

 in Echidna; and in others, as Raia, it abuts against the neural 

 axial canal, thus becoming a really complete shoulder-girdle. 



The humerus we have found to be a bone constantly single 

 in all Vertebrates above Fishes at the least. But the relations 

 of size of its tuberosities may be reversed, and the so-called 

 lesser, i.e. the ulnar tuberosity, may be the larger one. 



The elbow-joint is directed backwards ; but when the 

 rudimentary limb first appears, the arm is so placed that 

 the joint would be directed outwards. Later, the elbow be- 

 comes rotated backwards. 



When this rotation is effected the palm of the hand would 

 be directed forwards (i.e. pre-axially), but that by the move- 

 ment of pronation it becomes directed backwards (i.e. post-, 

 axially) or downwards as when we rest the palm of the hand 

 on a table. 



19. In the primitive, embryonic position, the back of the 

 humerus, the olecranon, and the back (or as it is called, from 

 its muscular relations, 1 the extensor surface) of the hand are 



1 See Lesson VIII. 



