Agnath medial fins appear to be at best only parallelisms 

 to those of other fishes. Fin rays as such are never developed. 



BILATERAL APPENDAGES 



Most vertebrates have two pairs of appendages, a pectoral 

 and a pelvic pair. These appendages and the girdles which 

 support them can now be described following the same 

 sequence set up in the earlier chapters. 



Mammal 



Pectoral appendage The girdle of the pectoral limb consists 

 of the scapula, its coracoid process, and the clavicle (Figure 

 6-45). In the cat the coracoid process is small, but that of 

 the opossum is conspicuous and shaped somewhat like a 

 crow's beak, from which it gets its name. The coracoid proc- 

 ess of the opossum arises from a separate ossification center 

 which remains distinct even in the adult. The monotreme 

 (Figures 6-48, 6-72) has these three parts but they are more 

 strongly developed, and in addition it has a precoracoid 

 and interclavicle. An interclavicle may be indicated in some 

 placentals (rabbit Figure 6-72). Of these bones the clavicle 

 and interclavicle are dermal. 



The limb consists of a basal element, the humerus, which 

 articulates in the glenoid fossa formed by the scapula and 

 coracoid. Distally this basal element articulates with the 

 radius and ulna; the latter is extended past the joint as the 

 olecranon. Distal to these forearm elements are the carpals, 

 of which there are usually eight; occasionally a centrale oc- 

 curs. In man (Figure 6-46) these carpals are given names 

 somewhat different from those of comparative anatomv. 

 These names are listed in Table 6-2. 



Beyond the carpals are the metacarpals and the phal- 

 anges. Each digit has several phalanges from which the 

 formula 2-3-3-3-3, starting with the number in the first 



digit (the thumb), can be derived. This formula is common 

 to man, the cat, the opossum, and the monotreme — if one 

 counts the claw bearing element of these animals as one of 

 the phalanges. 



The limb and its make-up of bones varies according to 

 its use. The extremes are the flipper of the whale and the 

 forelimb of the horse. In the whale the number of digits is 

 usually reduced and the middle digits may have many phal- 

 anges. In this type of limb the basal element (humerus) and 

 the forearm are shortened. In the horse the middle (third) 

 digit and its metacarpal are much elongated, the second 

 and third metacarpals are vestigal, while the first and fifth 

 are lost. Among marsupials reduction in the number of di- 

 gits also is observed among kangaroos. 



Several features of interest are observed in this limb. 

 Most of the bones have epiphyses. The carpals lack these 

 and there are no epiphyses on the proximal ends of meta- 

 carpals II to V. This situation is matched in the opossum 

 with the difference that the distal epiphyses of the radius 

 and ulna remain distinct in the grown animal, whereas 

 those of the cat or man are indistinguishably fused into the 

 ends of these bones. 



In man there is a small sesamoid bone (type of bone 

 formed in a ligament or tendon) below the joint between 

 the metacarpal and proximal phalanx of the thumb. At 

 this joint of each of the five digits of the opossum, there is a 

 pair of sesamoids. The opossum also has a small prepollex 

 ossicle on the inner margin of the carpus at the radius- 

 radiale joint. In the make-up of the carpus, there is only 

 slight variation: the monotreme differs from the opossum or 

 cat in lacking the intermedium. 



Pelvic appendage The girdle of the pelvic appendage in 

 the human or cat consists of three elements which fuse to 

 form the innominate bones of either side. These three com- 

 ponents are the (dorsal) ilium, the (ventroposterior) ischium, 



clavicle' 



glenoid fossa 

 -coracoid coracoid 



B 



Figure 6-45. Pectoral girdle and limb of mammal. A, forelimb of Armadillo; B, scapula and cora- 

 coid of sloth; C, scapula and coracoid of opossum. 



BILATERAL APPENDAGES 



171 



