30 CLASSIFICATION 



a raven), has been named the coracoid process. Here again we 

 have a vestige of what is an important structure elsewhere, repre- 

 senting as it does the coracoid bone, which, in such a creature 

 as a bird, is as large as the scapula and quite distinct from it. 



i. Arm. The arm itself is supported by a number of bones, 

 many of which belong to the class of long bones, and of which 

 we may take the humemis^ or upper-arm bone, as a typical example. 

 This consists of a hollow marrow-containing shaft, and swollen 

 extremities presenting smooth gristle-covered surfaces w^ell adapted 

 for entering into the formation of joints. The shaft is a good 

 example of the mechanical device of the hollow column, presenting 

 the advantages of lightness and economy of material without loss 

 of strength. The thickened ends are made for the most part of 

 spongy bone, consisting of layers arranged in such a way as to 

 combine great strength with lightness, while at the same time the 

 transmission of shocks is very largely hindered. As already 

 mentioned, the upper end of the humerus helps to form the 

 ball-and-socket-like shoulder-joint. At its lower end is a pulley- 

 shaped surface, which, in combination with the two long bones of 

 the forearm, gives the hinge -like character to the elbow -joint. 

 These bones are known as the radius and ulna, the former being 

 on the thumb-side, and the latter on the little finger side. The 

 upper end of the ulna is large, projecting behind the elbow as 

 what is popularly called the "funny bone", but is in reality not 

 a separate bone at all. The lower end of the ulna is comparatively 

 small. Exactly the reverse is true of the radius, which is small at 

 its upper end, but large at its lower end, where it has to support 

 the hand. And here we find a point of great interest. The fore- 

 arm is capable, as is known to everyone, of a rotatory movement 

 whereby the hand can be turned with its back upwards, the 

 position of pronation, as contrasted with the position of supination, 

 in which the palm is upwards. In the latter position the ulna and 

 radius are parallel, but in the former the radius, carrying with it 

 the hand, crosses obliquely over the ulna. Such an arrangement 

 obviously adds vastly to the use of the hand and arm, and is 

 absent in those animals where the fore-limbs are used mainly for 

 locomotor purposes. 



The wrist is supported by eight small irregular bones, arranged 

 so as to combine strength with flexibility, as well as to break the 

 shocks to which the hand is especially liable. They are succeeded 



