THE ENDOSKELETON 177 



tions, and it is also true that as a rule it is unsymmetrical and 

 not symmetrical forms that show the early conditions of a 

 part. 



Immediately above the fishes the chiropterygium or hand- 

 form appears, a type so definite and fixed in character that all 

 limbs from the amphibians on may be directly referred to it, 

 while it appears in almost its typical condition in animals 

 widely separated. (Fig. 47.) "It consists of a proximal joint 

 formed of a single bone, a second joint of two, followed by a 

 series of several small pieces from which extend five digits, 

 each composed of several bones. Unfortunately the parts were 

 originally named without much reference to the striking cor- 

 respondence (serial homology) between the anterior and pos- 

 terior limbs, and thus some of the corresponding parts have 

 received very different names, leading to a redundancy of 

 terms. The nomenclature and correspondence are indicated 

 in the following table : 



Anterior Limb. Posterior Limb. 



Humerus Femur 



Ulna (outer) Fibula (outer) 



Radius (inner) Tibia (inner) 



Carpus Tarsus 



Metacarpus Metatarsus 



Phalanges Phalanges 



The digits are designated either by number, I-V, or else 

 the Latin names for the fingers are used, pollex, index, medius, 

 annular is, minimus. These names are applied equally to both 

 members, except that for digit I of the posterior limb the term 

 hallux is employed instead of pollex. 



The nomenclature of the bones of carpus and tarsus has 

 caused by far the most difficulty, as they are extremely vari- 

 able and liable to fuse with one another or to disappear. Still, 

 in spite of much irregularity, they are reducible to a type or 

 pattern, as given in the diagram, to which the individual cases 

 may be referred. In this typical form (Fig. 47, A), there is 

 a piece at the distal end of each of the two limb bones of the 

 second joint, and five at the proximal ends of the five meta- 



