58 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



nearer the urostyle or vice versa when both muscles of the two sides act together, 

 or turns the back to one side, when acting singly. 



(5) The coccygeo-iliacus is the diagonal muscle parallel to and behind the 

 preceding running from the posterior two-thirds of the urostyle to the ilium. 

 Action, fixes the urostyle with respect to the pelvic girdle. 



c) Muscles of the sides of the abdomen: 



(1) The external oblique is the large muscle covering the sides of the abdomen 

 extending from the dorsal fascia and the ilium to the linea alba. Its fibers run 

 diagonally backward. Action, to support and reduce the abdominal cavity, and 

 to cause exhalation in lung breathing. 



(2) The transverse (including the internal oblique of other vertebrates) lies 

 under the preceding, its fibers running diagonally forward. Strip off the external 

 oblique cautiously to see it. Relations and function similar to preceding. 



d) Muscles of the ventral side of the trunk: 



(1) The rectus abdominis is the flat segmented muscle lying on both sides 

 of the median ventral line, extending from the pubic symphysis to the sternum. 

 Action, supports the abdominal contents, and fixes the sternum in place. 



(2) The pectoral muscle is the large muscle of the anterior ventral part of the 

 body. It arises from the various parts of the sternum and from the lateral 

 border of the fascia of the rectus abdominis muscle. It is inserted on the deltoid 

 ridge of the humerus. Action, draws the arm toward the ventral side and leg 

 (adductor of the arm) and expands the abdominal cavity. 



e) Muscles of the floor of the mouth and the hyoid apparatus: 



(1) The mylohyoid is a thin sheet of muscle running crosswise from one half 

 of the lower jaw to the other along their whole extent. It forms the outermost 

 layer of the buccal floor and its function is to raise this floor in the breathing 

 movements. 



(2) The submental muscle. Cut through the mylohyoid muscle in its median 

 ventral line and deflect the two halves. Under it at the very anterior tip of the 

 lower jaw is the small submental muscle. Its contraction pushes the sublingual 

 tubercle upward against the premaxillary bones of the upper jaw and thereby 

 closes the external nares in lung respiration. 



(3) The geniohyoid muscle comprises the longitudinal bands revealed by the 

 removal of the mylohyoid, consisting of medial and lateral portions arising 

 under the submental muscle and lateral to it from the lower jaw, and extending 

 to the postero-lateral processes of hyoid and the body of the hyoid as far as the 

 forking of the thyroid processes. Action, pulls the hyoid apparatus powerfully 

 forward, thus raising the floor of the buccal cavity in respiration; also helps 

 to swallow, to open the mouth, to lower the tip of the jaw, thus opening the 

 nares, and to move the tongue. 



(4) The sternohyoid is a continuation forward of the rectus abdominis, 

 extending from the underside of the coracoid and clavicle to the body of the 



