THE MUSCULAR SYSTEM 



269 



muscles are thus seen to be derivatives of lateral trunk muscles. Differen- 

 tiation of the muscles thus formed takes place in two directions in elasmo- 

 branchs and higher animals. First, the appendicular muscles are 

 subdivided into intrinsic muscles which lie within the fin and extrinsic 

 muscles which are connected with the fin but lie within the body-wall. 



LEVATORES ARCUORUM Cl-7) 



A. 



VISCERAL SKELETAL ARCHES Cl -7) \ 



DEPRESSORES ARCUORUM Cl-7) 



LEVATORES f-4 



DIGASTRICUS 

 MASSETER 

 TEMPORALIS 



DORSO-LARYNGIS AND 

 DORSO-TRACHEALIS 



B 

 IMTERMANDIBULARES |/ RYO-LARYNGEI 



HYO-PHARYNGEI 



Fig. 223. — Diagrams illustrating the hypothetical evolution of the branchiomeric 

 muscles. A. Hypothetical ancestral form. B. Branchiomeric muscles in urodele 

 amphibian. (Redrawn after Wilder's "History of the Human Body," Henry Holt 

 & Co.) 



Both groups are subdivided into levators and depressors. On the anterior 

 side of the fin, a muscle is formed which pulls the fin forward towards the 

 head. No special antagonistic muscle is differentiated in elasmobranchs, 

 the adduction of the fin being effected by the combined action of the 

 posterior part of the levator and depressor groups acting together. 

 The extension of the extrinsic muscles of the fins in fan-Hke form over the 

 lateral trunk muscles tends to obscure the metamerism of these in the 

 region of the appendages. The trapezius muscle which extends from 



