690 THE SKELETAL SYSTEM 



Structures for the protection of blood vessels are developed in the interver- 

 tebral area. 



d. Development of the Appendicular Skeleton of the Paired Appendages 



1) General Features. Two types of appendages are found in the vertebrate 

 group: 



( 1 ) median unpaired appendages which take their origin in the median 

 plane and 



(2) paired bilateral appendages which arise from the lateral surface of 

 the body. 



Median appendages appear in the fishes, aquatic urodeles, and in the larval 

 form of anuran amphibia. They also occur in the crocodilian and lizard 

 groups, among the reptiles, and, among mammals, in the whales. 



All appendages arise as outgrowths of the body. The median appendages 

 or fins of fishes possess separate skeletal structures for support, but the 

 median, fin-like structures in the tails of amphibia, reptiles, and whales do 

 not acquire a separate internal skeleton. All fishes possess a median caudal 

 or tail fin at the terminus of the tail, a median anal fin posterior to the anal 

 area, and one or more median dorsal fins. 



Most vertebrates possess two pairs of bilateral appendages (Chap. 10, 

 p. 508), one pair located anteriorly in the pectoral or breast region and the 

 other pair situated posteriorly in the pelvic area just anterior to the anus. 

 Each paired appendage has a skeleton composed of two parts: 



( 1 ) a girdle component and 



(2) a limb component. 



The girdle component of each appendage is associated with the axial skele- 

 ton of the trunk and also with the girdle component of the appendage on 

 the contralateral side. The entire girdle of each pair of appendages thus tends 

 to form a U-shaped structure with the closed portion placed ventrally (fig. 

 323 A-K). In fishes, the open dorsal area of the U-shaped girdle in the pec- 

 toral area may be closely associated with the axial skeleton, but, in land 

 forms, it is the pelvic girdle which joins the axial skeleton. This relationship 

 is to be expected, for, in fishes, the tail is the more important propulsive 

 mechanism, the head region being the "battering ram" insinuating itself 

 through the water. As a result, the skull, anterior vertebrae, and the pectoral 

 girdle ofttimes form a composite structure as, for example, in many teleost 

 fishes. In land-living vertebrates, on the other hand, the main propulsive force 

 is shifted anteriorly from the tail region and is assumed to a great extent by 

 the posterior pair of appendages. In consequence, the pelvic girdle acquires 

 an intimate relationship with the axial skeleton, and a fusion of vertebrae to 

 form the sacrum occurs. The sacrum serves as the point of articulation be- 



