1925] External Anatomy of Head and Abdomen of the Roach 201. 
passes above the tentorial plates itn and etn of Fig. 13, and out 
of the head into the neck through the occipital foramen ocf. 
The mouthparts, as was pointed out a century ago by Sav- 
igny, are modified limbs. The mandibles represent the basal 
segment of a mouth-part-limb (see Crampton, 1921), and their 
“cutting”or masticatory surface bears the incisors distally — i. e. 
the tooth-like projections in of Figs. 12 and 4, while in the middle 
of the median or masticatory surface is the mol a mo or grinding 
area. Basal to this is the submola or “accia” smo, an area lack- 
ing in pigment, and clothed with fine hairs in certain insects. 
The mandible has two “rocking points” or fulcra used as points 
of support when the muscles attached to the gnathotendons or 
mandibular tendons ft and et of Fig. 12 open and shut the man- 
dibles which work with a “sidewise” or “horizontal” movement 
as they meet under the upper lip Ir of Fig. 5. Thus, when the 
muscle attached to the flexor tendon ft of Fig. 12 contracts, the 
mandible rocking on the pivotal point h (Fig. 12) on one side, 
and on the pivotal point g (Fig. 4) of the other side is flexed 
or closed. On the other hand, when the flexing or closing mus- 
cles relax, and the opening or extensor muscles attached to the 
tendon et contract, the mandible pivoting upon the same two 
rocking points, is opened, or extended. The flexor tendon ft is 
borne by a small plate gn (Fig. 12) in the medio-basal region of 
the madible, and the flexor tendon is usually hugely developed to 
furnish points of attachment for the powerful muscles which 
close the mandibles in the movements of chewing. Since the 
opening muscles need not be very powerful, their tendon et, 
which is attached to the protuberance gpr, is not very large. Of 
the two rocking or pivoting points, the posterior or ventral 
one h of Fig. 12 consists of a condyle which is received in a cup 
or theca pgt 9 (Fig. 12) at the anterior (ventral) end of the post- 
gena pge (compare also Fig. 11). The anterior or dorsal pivot- 
ing point of the mandible is a projection labelled g in Fig. 4, 
which is received in a theca cth at the postero-lateral angle of 
the clypeus (compare also Fig. 5). The process g of Fig. 4 bears 
a groove which fits over a ridge or projection of the clypeal angle, 
and is thus a kind of condyle and cup combined. 
The maxilla (Fig. 2) represents a modified mouth part-limb, 
