THE MUSCULAR SYSTEM 199 



and is still crossed by the myocommata, that mark the original 

 elements, but as the higher forms are reached the myocommata 

 disappear, and the primary segmental nature of the muscular 

 sheet is shown only by the distribution of nerves and blood 

 vessels. In the rectus abdominis the same thing has taken 

 place, but here the effacement of the myocommata is not com- 

 plete, and a few of them are still present, even in mammals, 

 forming the " tendinous interscriptions " of human anatomy 

 (3-4 in man). 



III. Extension over a much larger area of an element origi- 

 nally belonging to one or two myotomes. 



This is seen especially well in the case of certain of the 

 muscles of the anterior limb, notably the pectoralis and the 

 latissimus dorsi. Small and not very extensive in the earliest 

 land animals, these muscles increase with the importance of the 

 limbs to which they belong, and may eventually extend over 

 a large number of myotomes. In their final condition such 

 muscles closely resemble such a sheet as that of the external 

 oblique of the abdomen, but their composition is very dif- 

 ferent, the one being the result of the fusion of elements de- 

 rived from several myotomes, the other the extension of an 

 element derived from one or a few. 



IV. Further extension of a pair of muscles that have come 

 to meet in the median line through the formation of a median 

 skeletal crest or keel. 



The two most conspicuous examples of this are the pector- 

 alis muscle on the ventral side of the thorax and the tem- 

 poralis of the mandible. Of these, one is originally an appen- 

 dicular, the other a visceral muscle, each probably a derivative 

 of a single myotome. Both extend their area of origin with 

 their increase of function and size, the one dorsally over the 

 sides and top of the skull, the other ventrally over the chest. 

 Further progress being stopped by the meeting of the two 

 muscles in the median line, they obtain an extended point of at- 

 tachment through the formation of a median ridge. The most 

 excessive instance of this is seen in the enormous keel of the 

 sternum in certain birds, notably the humming-bird. 



