THE MUSCLES 55 



The M. interhyoideus is a Vllth nerve muscle, however, and its 

 connexion with the mandible is secondary. 



The M. intermandibularis posterior arises from the anterior two- 

 thirds of the mesial upper edge of each ramus of the mandible except 

 for a very short distance near the middle line anteriorly, which latter 

 gives rise to the M. genio-glossus. The fibres run transversely across 

 the space between the two rami of the lowerjaw, but are interrupted in 

 the middle line by a wide aponeurosis of tough transparent connec- 

 tive tissue, the linea alba. In a fresh specimen the MM. genio- 

 hyoideus and genio-glossus are seen through this aponeurosis and 

 the M. intermandibularis looks as though it were inserted into their 

 edges. 



Innervation : From N. mandibularis (R. intermandibularis). 



Function: The contraction of this muscle serves to elevate the 

 mouth floor, and it is therefore of service in breathing and degluti- 

 tion. 



M. levator bulbi (^\. IX, fig. ^2)- Hoffmann (1873-8); Driiner 



(1901); Burkard (1902); Luther (1914); Edgeworth (1920 et seq.). 



Fronto-pterygoidien ...... Duges (1834). 



An excellent description of this muscle has been given by Luther 

 (i 9 14) and his nomenclature has been adopted here. His anatomical 

 results, as far as the adult is concerned, are accurate, but his conclu- 

 sions regarding the phylogenetic history of the muscle do not agree 

 with those of Edgeworth. 



It is by no means easy to determine the direction of the fibres 

 in the several portions of this muscle, but the application of a little 

 dilute saffranin is of material assistance. 



The muscle consists of a thin sheet forming an elastic floor to the 

 orbit and lies between the bulbus oculi and the roof of the mouth, 

 ventral to the MM. rectus and obliquus inferior. Luther distin- 

 guishes the following three sections. 



{a) ThQ pars principalis (m.l.b.p.) is the most dorsal portion, lying 

 nearest to the bulbus oculi. It arises as a thin, tough, tendinous sheet 

 from the lateral borders of the parietal^ frontal^ and pre-frontal ele- 

 ments of the skull as far as the cartilago antorbitalis of the nasal capsule. 

 The fibres run mainly in a postero-lateral direction and converge so 

 as to form a more or less scimitar-shaped plate. They are inserted 

 on the ventral side of the posterior end of the lower eyelid (mem- 

 brana nictitans). 



(J?) The pars sagittalis arises from the palato-quadrate at the pos- 

 terior wall of the orbit, and may be further subdivided into two heads. 



