No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 547 
from the under surface and outer edge of the squamosal ; 
behind and below they arise from the entire outer, anterior 
face of the preoperculum, excepting only its outermost edge 
which gives attachment to the dermis covering the side of 
the head. A few fibres often arise from the under surface of 
the upper postorbital bone along its upper edge. Its deeper 
fibres arise from the outer surfaces of the hyomandibular, 
quadrate, and symplectic, the surface of origin lying immediately 
in front of the preoperculum and extending somewhat under 
the lower end of that bone and forward onto the hind edge of 
the mandible. This inner, deeper portion of Az is intimately 
connected, particularly below, with the inner, deeper portion, A, 
of the adductor. A part of Az, often found as a somewhat 
separate bundle, arises from a slightly raised portion of the 
outer surface of the hyomandibular immediately behind the 
middle portion of that bone. This surface is well defined above 
by the lower, nearly horizontal, border of the dilatator operculi ; 
in front by the line of attachment of the strong metapterygoid 
membrane, which extends across the open space between the 
hyomandibular and the median process of the metapterygoid 
and covers the insertion of the posterior portion of the levator 
arcus palatini ; behind by the front edge of the preoperculum ; 
and below by the external opening of the facial canal through 
the hyomandibular, and by the slight groove extending from 
this canal downward and backward to the point where the hyoid 
branch of the facial nerve passes under the preoperculum. It 
is below and in front of this groove that the superficial muscle 
Az is, at its origin, so intimately connected with 43. 
Although continuous and single at its origin, the superficial 
muscle, 42, always shows, toward and at its insertion, indications 
of a separation into three portions, a lower or posterior one, A2’, 
a middle one, 42”, and an upper or anterior one, A2'’. The 
middle portion, A2!’, sometimes shows a further but much less 
marked separation into two parts, as shown in Fig. 40. Between 
the three main portions, at their insertion, the surfaces of sepa- 
ration are clean and distinct, the actual separation of the mus- 
cle into three parts is, however, wholly artificial. The surfaces 
of separation lie in the direction of two planes intersecting at 
