THE MANDIBULAR AND HYOID MUSCLES OF MAMMALS. 585 
mandible. The zygomatico-mandibularis-masseter is partially 
marked off from the external fork of the temporal by a groove 
passing upwards from the posterior edge. 
In a 23 mm. embryo (figs. 49-53) the muscles have become 
quite separate from one another. The internal pterygoid now 
Fig. 70. 
arises by two heads, one from the under-surface of the ala 
temporalis, the other from the palatine portion of the common 
Anlage of the palate and pterygoid bones; they unite and 
are inserted into the mandible. The anterior end of the 
tensor veli palatini winds round the hamulus into the soft 
palate; the muscle extends backwards medial to the internal 
pterygoid and is inserted partly to the tympanic bone, partly 
to the proximal end of the tensor tympani. The tensor 
tympani now lies wholly behind the internal pterygoid and 
tensor veli palatini owing to disappearance of its anterior 
