78 A MANUAL OF ANATOMY. 



middle and upper thirds and the second from beneath the 

 muscle at its middle. They run downward and backward 

 to disappear in front of the trapezius muscle. 



Digastric. Figs. 1 6, 17. 



Origin. Posterior belly, from the digastric groove in the 

 temporal bone. Anterior belly, from the under surface of 

 the lower jaw a little external to the symphysis. 



Insertion. The two bellies are joined by a central tendon, 

 which is fastened to the body and greater cornu of the 

 hyoid bone by a process of the deep cervical fascia. This 

 fascial loop is lined by synovial membrane. 



The tendon also passes through the stylohyoid muscle, 

 divides the insertion of that muscle, and by it is aided in 

 keeping its place. 



Nerve Supply. The posterior belly, by the facial. See 

 facial nerve, page 61. 



The anterior belly by the mylohyoid branch of the 

 inferior dental, which is a branch of the inferior maxillary 

 nerve. 



Action. (i) Posterior portion. Raises and retracts the 

 hyoid bone (and the structures attached to it). 



(2) The anterior portion. Depresses the lower jaw. 

 Acting from above, it will raise and draw forward the hyoid 

 bone. 



(3) Both portions acting will elevate the hyoid bone (and 

 the parts connected to it, especially the tongue). Hence 

 the significance of raising the lower jaw upward and forward 

 in anaesthesia, to prevent the tongue falling backward and 

 suffocating the patient. 



DISSECTION. 



Divide the sternomastoid muscle and the external jugular vein and reflect 

 their extremities. (Always divide veins between two cords tied firmly around 



