196 CHARACTERS OF VERTEBRATE ANIMALS 



illustration may be made to illustrate very well another biological 

 principle, i.e. " change of function ", already alluded to in the 

 Introduction (p. 13). A room intended for a particular purpose 

 may be, of course, used for some other purpose, as when what 

 should be a study is converted into a nursery. An amusing case 

 is known to the writer of a small tradesman, who, on retiring, 

 thought of taking a house of more ambitious nature than the one 

 that had been connected with his shop. Seeing a bath-room for 

 the first time, he remarked that he had no use for it, and would 

 make it a lumber-room. 



Some meaning will now be attached to the expression 

 "generalized type" as applied to the limbs of a lizard. But we 

 may go a step further, and speak of a " theoretical type", by which 

 is meant an arrangement so generalized that it is rarely, if ever, 

 found in existing animals. The endoskeleton of the limbs of 

 terrestrial vertebrates is best understood by reference to such 

 theoretical types, which may here be conveniently explained. 



As has already been pointed out (see pp. 29 and 145), the 

 skeleton of either fore- or hind-limb consists of a girdle by 

 which attachment to the trunk is brought about, and the skeleton 

 of the free limb, and it has also been indicated (p. 24) that there 

 is a correspondence or serial homology between fore- and hind-limb. 



Girdles. — The theoretical type of shoulder-girdle consists of a 

 dorsal piece, scapula, and two ventral pieces, the larger, coracoid, 

 behind, and the smaller, precoracoid, in front. At the junction of 

 the scapula with the other elements is a shallow glenoid cavity, to 

 which the bone of the upper arm is attached. The hip-girdle 

 consists similarly of three pieces — a dorsal ilium, uniting below 

 with a ventral and anterior pubis, and a ventral and posterior 

 ischium. Corresponding in position to the glenoid cavity is a 

 somewhat deeper cup, the acetabulum, for the attachment of the 

 thigh-bone. 



Free Limb. — If the limb of a lizard be spread out at right 

 angles to the body, in what is usually called \h& primitive position, 

 and a line or axis drawn down its centre, an anterior or preaxial 

 edge can be distinguished from a posterior or post-axial edge, 

 and an upper or dorsal surface from an under or ventral surface. 

 Taking first the fore-limb, we find an upper-arm bone or humerus, 

 down the centre of which the axis runs, and succeeding this in the 

 fore-arm, two bones, a preaxial radius, a post-axial ulna. Next 



