MYOLOGY 33 



M. sternomastoideus (figs. 7, 23, 24) is usually per- 

 fectly separable into two divisions in the case of Homo- 

 dontomys. The more superficial of these arises from half 

 the cranial border of the manubrium : the deeper one arises 

 similarly, deep to the first. Both are inserted, mostly by 

 tendon fibers, onto the mastoid process of the squamosal. 



In Neotoma the muscle is usually inseparable except at 

 the insertional end, and is slightly broader and heavier. 

 In Teonoma it seems to be really separable at no point, and 

 is also slightly larger than in Homodontomys. 



M. cleidomastoideus (figs. 6, 7, 23, 24, 29) has a ten- 

 dinous origin from the mastoid process of the squamosal, 

 with insertion upon the clavicle deep to the clavotrapezius. 

 In Teonoma this muscle is somewhat broader and coarser. 



Both of the foregoing muscles are served by the N. 

 accessorius. 



2. SuPKA- AND Infrahyoid group 



Under this heading belong 



Mm. omohyoideus ' thyrohyoideus 



sternohyoideus geniohyoideus 



stemothyroideus 



M. omohyoideus (figs. 7, 8, 28) is very thin and fragile, 

 but comparatively broad, and arises from the basal portion 

 of the suprascapular notch. It passes superficial to the 

 sternothyroid, partly between the digastric and the sterno- 

 hyoid, and is inserted deep to the insertion of the latter 

 muscle upon the hyoid bone. 



In Neotoma and Teonoma this muscle is much thicker 

 and better defined. 



M. sternohyoideus (fig. 7) is the chief muscle lying 

 immediately ventrad of the trachea. Origin is from the 

 first costal cartilage. Passing craniad adjacent to its 

 fellow of the opposite side, insertion is onto the body of 

 the hyoid bone. 



Li 

 I- 



