106 
Mouthparts of Pediculus 
and runs obliquely downward and backward to its insertion in the roof of 
the pharynx nearly in the middle line and partly under and in front of the 
anterior sphincter, (b) Medial dorsal. Each muscle arises from the roof of the 
epicranium posteriorly and runs downward to its insertion in the roof of 
the pharynx immediately behind the anterior sphincter, (c) Post-dorsal. 
These run almost parallel to the medial dilators. Their origins are close 
behind those of the medials but their insertions are under the medial 
sphincter. The function of these three pairs is to dilate the pharynx by raising 
the sagging roof. The anteriors probably assist in protracting the pharynx 
during feeding while the other two are retractile. ( d ) Lateral (l.d., Text-fig. 
Ill). Each is a very thick muscle which originates from in front of and 
above the eyes, and is inserted widely and strongly in the upper half of the 
wall of the pharynx at its middle. ( e ) Post-lateral. Each is a thick muscle 
which originates from the floor of the head posterior to the eyes and runs 
forward, upward and inward to the insertion in the lower half of the wall 
of the pharynx between the medial and posterior sphincters. The function 
of both pairs is to bulge the walls outward, and the post-lateral probably 
retract the pharynx after feeding. 
This account is at variance with Harrison’s in which a third dilator and 
sphincter are not mentioned. 
Suction seems therefore to take place as follows: the haustellum is 
placed in close contact with the host’s skin and, with the assistance of the 
sac tube, air is prevented from entering the food canal. The contraction of 
the dilators of the pumping-pharynx causes a negative pressure in the latter 
and the blood is drawn in from the wound. The anterior sphincter controls 
the entrance to the pharynx, the medial regulates the supply, while the 
posterior controls the egress. The anterior dilators of the pharynx contract 
and raise the anterior portion of the sagging roof while the anterior sphincter 
is relaxed. Blood thus enters the anterior part of the pharynx. The anterior 
dilators then relax and the anterior sphincter contracts and blood is retained 
in the centre of the pharynx as the medial, dorsal and lateral dilators contract 
and increase the volume of that organ. The medial sphincters next relax and 
the post-dorsal and post-lateral dilators contract and cause blood to enter 
the posterior part of the pharynx; this of course taking place at the same 
time as the medial and lateral dilators relax and the medial sphincters con¬ 
tract. The last effort is the relaxing of the posterior sphincter to permit 
blood to enter the oesophagus, this happening when the medial sphincter 
contracts and the post-dorsals and post-laterals relax. The whole process 
recalls that of peristalsis. I have not observed the occurrence of re-gurgitation. 
Oesophagus. The pharynx opens into the gullet which is a thin-walled 
tube, curving upwards in the middle line and running into the thorax where 
it opens into the stomach. 
Stabber-Sac. It is convenient to consider first the relationships of the 
various structures of the region where food canal and stabber-sac diverge. 
