Tus. Hoc Louse 677 
In cleared specimens he evidently saw only the anterior, strongly chitinized 
band of the pharynx. It is a somewhat cone-shaped structure having its 
widest diameter, which is approximately 0.15 millimeter, a little posterior 
to its transverse median line. In sections its ventral aspect is seen to 
lie almost level along the median longitudinal line of the head, and its 
dorsal surface passes obliquely toward the top of the head. Between its 
transverse median line and its part of greatest diameter is a more strongly 
chitinized region crossing the dorsal surface as a band and passing obliquely 
and posteriorly down the sides to the ventral surface, where the two 
bands run backward for a short distance, each lying somewhat laterad 
of the median line (Plate LX,3). Behind the muscle insertions is a second 
region of strong chitinization, followed by a sphincter muscle, behind 
which the diameter lessens until it passes as the slender esophagus under 
the brain. 
The esophagus 
The esophagus (Plate LX, 2 and 3) passes directly backward between 
the tritocerebral lobes of the brain, over the sub-esophageal ganglion, 
and into the thorax between the two main tracheal trunks. At the 
posterior end of the head the esophagus, the dorsal vessel, the tracheae, 
and the connectives between the sub-esophageal and thoracic ganglia, 
are inclosed by a wall of thin cuticula, which is continuous with and shows 
the same staining reactions as the cuticula separating the posterior end 
of the piercer sheath from the thorax. It is a structureless membrane 
(Plate LX, 5). At its posterior end the esophagus passes over the anterior 
part of the stomach lying in the thorax, and enters its dorsal surface 
under the tergite of the second abdominal segment. Its length from the 
posterior end of the true pharynx to its passage into the stomach is 
approximately 1.03 millimeters and its diameter 0.03 millimeter. In 
sections its wall is seen to consist of flattened epithelial cells lined by a 
thin chitinous intima, but no basement membrane can be distinguished. 
The usual muscle layers are present, but are so fine as to be distinguished 
only with considerable difficulty. At rest and empty, as it is seen in 
sections, the wall shows a number of small convolutions. 
The ‘‘ mandibles’ of Enderlein 
On either side of the pumping pharynx, where the posterior arms of 
the buccal plate fuse with its lateral walls, lie two triangular chitinous 
