Motor System — Muscles and Skeleton 



129 



*CT|N0TRICH1A PR0PTERYG1UM 



MESOPTERYCIUM I ffl 



M 



SUPRASCAPULA 



Fig. 122. Diagram of the pectoral appendage of a shark. (Courtesy, Neal and 

 Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



with corresponding regions of a tetrapod girdle, the ventral bar is 

 called the coracoid and each dorsal projection is a scapular process. 

 At the junction of each scapular process with the coracoid, the mov- 

 able fin is attached. The surface on the girdle where the fin articulates 

 is the glenoid facet. The skeleton of the fin consists of three basal 

 plates of cartilage, an anterior propterygium, a mesopterygium, 

 and a posterior metapterygium. Distal to these are numerous carti- 

 laginous fin-rays (radialia), each divided into several parts. The thin, 

 bladelike distal part of the fin is supported by a system of parallel 

 coarse fibers consisting of an elastic hornlike substance produced by the 

 dermis. The pelvic appendage is not so strongly developed. The 

 girdle is just behind the coelom or at the base of the tail. Its transverse 

 part is called the "ischiopubis" and the dorsal process at each end of 

 it is the iliac process. The facet for articulation of the fin is the ace- 

 tabulum. The fin, smaller than the pectoral, may have only one or 

 two basal cartilages. 



The appendicular skeleton of bony fishes is, in its general 

 plan, like that of sharks, but in the adult it is more or less completely 

 ossified (Fig. 123). In the pectoral girdle, the embryonic coracoid 

 cartilage may consist of right and left parts not joined midventrally. 

 Centers of ossification arise by pairs, producing on each side a ventral 

 coracoid bone and a dorsal scapula. The cartilaginous girdle is 

 usually more or less reduced and functionally replaced by membrane 

 bones. Of these, the largest and most commonly occurring are a pair 

 of ventral clavicles which supplement the coracoids and a pair of 

 cleithra situated dorsolateral to the clavicles. The two clavicles may 

 meet midventrally, thus restoring the completeness of the ventral 

 arch. The girdle may be augmented by other membrane bones, and 

 commonly a series of several of them connect the cleithrum with the 



