MUSCULAR SYSTEM 619 



inner angular process of the mandible, acting on the jaw behind the 

 articulation so as to open the mouth. 



M. tempvralis, consisting of a variable number of parts, the chief 

 of which, arising from the postorbital process and the quadrate, pass 

 beneath the jugal arch and are inserted on the mandible in front of 

 the joint, acting mostly as masseters. Two or three smaller muscles, 

 arising from the deeper region of the orbit and the interorbital 

 septum to be inserted on the palatal and pterygoid bones, are much 

 less constant. i 



M. pterygoideus, arising chiefly from the ventral face of the 

 pterygoid, palatal and, sometimes, from the maxillary bones, and 

 inserted on the inner face of the mandibular articulation, close the 

 bill or, when the mouth is open, flex the upper mandible, as seen in 

 Psittaci, Anatidse and others. 



3. Group of the Hyoid muscles, supplied solely by nervus hypo- 

 glossus often very numerous and always attached to the HYOip 

 apparatus (page 452), whence they reach backward to the sternum 

 or to the furcula as mm. sterno-hyoidei or mm. deido-Tiyoidei, to the 

 larynx and trachea as mm. thyreo-Jiyoidei or mm. tracheo-hyoidei, while 

 others extend forward to the mandible as mm. genio-Jiyoidei and genio- 

 glossus, or lastly they connect the various portions of the Hyoid 

 apparatus. In most cases their position is indicated by their name. 



System of the m. sterno-hyoideus, a long pair of muscles, pre- 

 senting its least differentiated condition in Apteryx (where no other 

 sterno-hyoid or sterno-tracheal exists). The broader and more 

 superficial portion arises from the ventral face of the thyroid 

 cartilage and the hyoid bones, meeting its fellow without being 

 attached to the trachea, and is inserted aponeurotically on the 

 lateral and posterior margin of the sternum, partly covering the 

 muscles of the shoulder and breast. The deeper portion likewise 

 begins at the thyroid cartilage, passes down the side of the trachea, 

 to which it is firmly attached until just above the bronchial fork, 

 where it leaves it to be inserted near the coraco-sternal articulation. 

 From the conditions just described are differentiated the more 

 complex arrangements found in other Birds. By reduction of the 

 muscular mass about the middle of the neck an upper and lower 

 portion are formed, the upper then appearing as tracheo-laryngeal or 

 thyrohyoid muscles the lower as sterno- or cleido-tracheal, and 

 through further extension to the bronchi as muscles of the SYRINX. 

 In many Birds the superficial portion of the whole system remains 

 as one or two ribbons, m. cleido-hyoideus, running along the side of 

 the neck and connecting the tongue with the furcula, or other parts 

 of the scapular arch. The chief retractor of the tongue, M. tracheo- 

 hyoideus reaches its highest development in some of the Picidx, where 

 it takes several spiral turns round the trachea. 



M. genio-hyoideus, arising from about the middle of the mandi- 



