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. 



FIG. 97. LATERAL MUSCLES- OF Ainphioxus. 



D, dorsal, and 7", ventral portions ; M, J/, the individual myotonies ; QB, transverse 

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



Mo 



JLM 



T 



Da 

 / Me* V 



Ma 



FIG. 98. THE ENTIRE MUSCULATURE OF Siredon pisciformis. 



LI, lateral line ; D, dorsal, and.F", ventral portion of caudal muscles; -ZZJ/, 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 (<>!,) ; 

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

 beginning of the differentiation of a rectns ahdominis ; at // ' the reetns-system 

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

 tomes of the dorsal portion of the lateral museh-s ; 7\ temporal : J/</, masseter : 

 Dg, digastric; .17// 1 , mylohyoid (posterior portion): <'>\ external ceratnhynid 

 muscle; Lv, levator arcuum branchialium ; fH-, Irvator branchiarum ; < '/>//, 

 cervical origin of the constrictor of the. pharynx ; Th, tliymus gland 



latissimus dorsi ; !>*, dorsalis scapulae ; i'n, cucullaris ; 

 procoraco-humeralis. 



/./, 

 1 , snprascapula ; /'//, 



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,- 1 except in the caudal region, 



