No. 468.) MUSCLES OF ACANTHIAS AND RAIA. 917 
ward and backward to the lower jaw where they are attached to 
the entire width of Meckel's cartilage. The details of the 
course of the fibers are rather difficult to express. On the 
lower jaw (Fig. 1) they extend nearer to the middle line than 
on the upper, and this portion (r) instead of coming from the 
fascia which separates it from the rest, comes from the posterior 
part of the upper jaw. A strong tendon is attached to this part 
which runs forward into the levator labialis superioris muscle. 
Other fibers are specialized in function. A broad thin sheet 
of fibers is extended on the upper half of the adductor and has 
its true origin from the under surface of the postorbital process, 
while a strong mass of tendinous material at the angle of the eye 
(Fig. 13, y) gives origin to some of the fibers. A few fibers of 
the first ventral constrictor, which have already been referred 
to as Csv Za, are attached to the outer surface of the adductor. 
Rata. 
In the skate the adductor muscles are more complicated. 
They cover and surround the lateral ends of the jaws and are 
divided into distinct and easily separated layers. Following 
Tiesing, they may be grouped into a small medial, and a larger 
lateral portion, the latter in turn divisible into deeper and super- 
ficial layers. 
Adductor Mandibularis Medialis— This, the smallest of the 
adductors (Figs. 3, 6, Amm) arises from the anterior edge of 
the upper jaw. Its fibers run back across the angle of the 
mouth and then medially to become inserted near the anterior 
(occludent) margin of the lower jaw not far from the symphysis. 
Its origin is in common with the deeper layer of the lateral 
adductor and the two separate where the levator labialis superi- 
oris passes to become attached to the fascia between them. 
The adductor mandibularis lateralis is subdivided into a deep 
layer (Figs. 3, 6, Am /,) and a more superficial portion ( Ami 2 
and Aml 2a). The deep layer is largely covered by the super- 
ficial. The majority of its fibers arise from the anterior edge of 
the upper jaw as well as from the entire outer surface of the 
muscle process, and they find their insertion upon the outer sur- 
face of the lower jaw about opposite the point of origin. 
