114 



COMPARATIVE ANATOMY. 



as to the segmentation of the axial skeleton, forms a character- 

 istic feature in Vertebrates, and distinct indications of it may be 

 traced in all representatives of the group up to Man. 



JIM v OB 



FIG. 97. LATERAL MUSCLES OF AmpMoxus. 



D, dorsal, and V, ventral portions ; M, M, the individual myotomes ; QB, transverse 

 muscles of abdominal region ; C, cirrlii ; F, F 1 , tail-fin. 



RM 



FIG. 98. THE ENTIRE MUSCULATURE OF Siredon pisciformis. 



LI, lateral line. ; D, dorsal, and .F, ventral portion of caudal muscles; RM, dorsal 

 portion of lateral muscles of the trunk ; 0, 0, outermost layer of the external 

 oblique muscle, arising from the lateral line, and extending to the fascia, F ; at * 

 a piece of this layer is removed, exposing the second layer of the muscle (Ob) ; 

 at Re the oblique fibres of the, latter pass into longitudinal fibres, indicating the 

 beginning of the differentiation of a rectus abdominis ; at Re 1 the rectus-system 

 is seen passing to the visceral skeleton ; Me, fibrous partitions between the myo- 

 tomes of the dorsal portion of the lateral muscles ; T, temporal ; Ma, masseter ; 

 Dg, digastric ; Mh l , mylohyoid (posterior portion) ; Go, external ceratohyoid 

 muscle ; Lv, levator arcuum braiichialium ; ftt, levator branchiarum ; Cph, 

 cervical origin of the constrictor of the pharynx ; Th, thymus gland ; Lt, 

 latissimus dorsi ; Ds, dorsalis scapulas ; Cu, cucullaris ; SS, suprascapula ; Ph, 

 procoraco-humeralis. 



The cranio-visceral musculature is to be looked upon as having 

 been derived out of the lateral muscles in consequence of the 

 development of the visceral skeleton. 



It may be asserted as a general rule that the dorsal portion of 

 the lateral muscles of the trunk, except in the caudal region, 



