MUSCOLAR SYSTEM 619 
inner angular process of the mandible, acting on the jaw behind the 
articulation so as to open the mouth. 
M. temporalis, 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. 
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, Anatidxe and others. 
3. Group of the Hyoid muscles, supplied solely by nervus hypo- 
glossus often very numerous and always attached to the Hyorp 
apparatus (page 452), whence they reach backward to the sternum 
or to the furcula as mm. sterno-hyoidet or mm. cleido-hyoidei, to the 
larynx and trachea as mm. thyreo-hyoidet or mm. tracheo-hyoidei, while 
others extend forward to the mandible as mm. genio-hyoidet 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- 
