220 F. H. EDGEWOETH. 



The anterior belly of a digastricus verus is innervated by the 

 Vth, the posterior belly by the Vllth. A dig-astricus 

 spurius has a deeper or superficial tendinous inscriptiou, 

 and is also innervated by both Vth and Vlltli. 



Anatomists Avho have investigated the adult conditions of 

 the muscle have expressed various views as to its origin; 

 some, e.g. His and Chaine, thought that the muscle was 

 developed from one mass; others, e.g. Humphrey, Gegen- 

 bauer, Riige, Fiirbringer, Bijvoet, have held that it was 

 developed from two masses. 



Bijvoet, the latest investigator of the subject, was of opinion 

 that the condition present in Monotremes — a M. depressor 

 mandibulae anterior, and a M. stylohyoideus — is the "Aus- 

 gangsforin," that from this was developed a digastricus 

 verus, and that a digastricus spurius was a secondai-y con- 

 dition. He was also of opinion that a M. intermandibularis 

 separated into a M. mylohyoideus and a M. depressor 

 mandibulte anterior. The only descriptions of the develop- 

 ment of the digastric muscle which have hitherto been given 

 are those by Futamura of human and pig embryos. He says 

 that the common Anlage of the digastricus, stylohyoideus, 

 and stapedius is visible in twenty-seven to thirty day human 

 embryos as a " medialwiirts sich anhiiufende dicht gedriingte 

 Muskelblastemgewebe," Avhicli is continuous laterally with the 

 platysma Anlage, the whole forming a single mass. In embryos 

 a little older the platysma and digastric Anlagen separate. In 

 six weeks' embryos the digastric Anlage passes forward to the 

 anterior l)order of the low^er jaw. The mylohyoid nerve at this 

 date passes to the mylohyoid muscle only ; it does not innervate 

 the anterior belly of the digastric until the age of seven weeks. 

 The stylohyoid and stapedius begin to separate from the 

 posterior ])elly of the digastric at the age of six weeks. The 

 digastric is thus solely a facial muscle, theanterior part of which 

 receives a secondary innervation from the trigeminal nerve. 



Bijvoet doubted this account given by Futamura, as it 

 appeared to him to disagree with the results of comparative 

 anatomy. 



