MYOLOGY 153 



the head. In the hmited interval between these two 

 muscles a part of the rhomboid and parts of other muscles 

 are seen with which we shall soon be occupied. 



This muscle, as regards the horse, is described by some 

 anatomists as consisting of two parts : one anterior, or 

 superficial ; the other posterior, or deep. In reality, the 

 first only corresponds to the mastoido-humeral, which 

 we are considering ; the posterior may be more exactly 

 regarded as representing a special muscle of quadrupeds, 

 but which is here a little deformed, the omo-trachelian (see 



P- 155)- 



When the mastoido-humeral contracts, taking its fixed 

 point above, it acts as an extensor of the humerus, and 

 carries the entire fore-limb forwards. If it takes its fixed 

 point below — that is to say, at the humerus — it inclines 

 the head and neck to its own side. If it contracts at the 

 same time as the mastoido-humeral of the opposite side, 

 then the head and the neck are carried into the position of 

 extension. 



The Sterno-mastoid (Fig. 66, 5 ; Fig. 68, it ; Fig. 69, 15 ; 

 Fig. 70, 17). — Having described the clavicular portion of 

 the sterno-cleido-mastoid in connection with the mastoido- 

 humeral, because it forms a part of the la.tter, we have, 

 in order to complete the homologies of this muscle, to study 

 now that which corresponds to its sternal portion. This is 

 the sterno-mastoid muscle. In all the quadrupeds with which 

 we are here concerned this muscle arises from the anterior 

 extremity of the sternum ; narrow and elongated in form, 

 it passes towards the head in a direction parallel to the 

 anterior border of the mastoido-humeral, from which it is 

 separated by an interspace which, along its whole length, 

 lodges superficially the jugular vein ; hence the name of 

 jugular groove, which is given to this part of the neck 

 (Fig. 10, 18). 



It is inserted, in the case of the dog and cat, into the 

 mastoid process, where it is united with the mastoido- 

 humeral ; in the ox it is divided into two portions — one 

 which goes to the base of the occipital bone, the other 

 passing in front of the masseter is b}^ the medium of the 



