538 MEDICAL ENTOMOLOGY 



time the saliva is conveyed downwards through the hypopharynx. The 

 dilator muscles of the buccal cavity, which are already brought into a 

 suitable position by the forward displacement of the chamber, then con- 

 tract, and bring about a negative pressure which draws up the blood 

 from the wound. While this is going on the sphincter between the 

 buccal cavity and the pharynx is closed. As soon as the buccal cavity 

 is full this sphincter relaxes, and the dilator muscles of the pharynx come 

 into action and withdraw the blood from the buccal cavity, the sphincter 

 posterior to the pharynx remaining closed. The blood flows in a straight 

 course to the pharynx, as the bend in the tube which connects it 

 with the buccal cavity is undone when the latter is thrust forwards. 

 When the pharynx is full the sphincter posterior to it relaxes and allows 

 the blood to flow into the oesophagus, probably with the assistance of 

 the muscles which pass from the dorsal to the lateral walls of the head, 

 the contraction of these bringing about an increase in the intra-cranial 

 pressure. When the feed is completed the buccal cavity and the 

 proboscis are replaced in position by means of their retractor muscles. 



The above account is of course very incomplete. One would like to 

 know, for instance, whether the maxillae alone are protracted during the 

 making of the wound, and to what extent the teeth at the mouth aperture 

 take part ; and how the food is transferred from the tube formed, as 

 seems so probable, by the maxillae, to the buccal cavity, with which it is 

 not directly continuous. 



In Haematopinus, which, according to Enderlein, is a more primitive 

 form than Pediculus, there is a pair of two-jointed mandibles in addition 

 to the maxillae. It is noticeable that in those lice which live on thick- 

 skinned animals, such as pigs and buffaloes, the proboscis is much longer 

 than in the human louse, and may extend into the anterior part of the 

 thorax. 



About the level of the pharynx there is a pair of glands which open 

 into the sheath of the proboscis, and probably represent salivary glands. 



They are simple and tubular, and are thrown into one 

 The glands of the 

 proboscis sheath or CO1 ^ S so that each may be cut several times in 



sections (Plate LXVII, fig. 3). They lie in the loose 

 tissue on each side of the sheath, and open into its lateral angles. The 

 walls are composed of a simple flattened epithelium set on a rather thick 

 basement membrane. Pawlowsky, who was apparently the first to 

 notice these structures, suggests that they furnish a secretion which acts 

 as an irritant in the wound, or else serves to lubricate the proboscis. 

 Their occurrence in this situation is of some interest, as they might 



