666 LAURA FLORENCE 
branch going to the disk must be the retractor muscle of the disk. 
Osborn figured this large muscle lying in the tibia as inserted in the dorsal 
wall of the tarsus, and a continuation passing from there to the dorsal 
curve of the claw. He also figured a flexor muscle of the tarsus. Neither 
of these two conditions has been found in the present investigation, and 
the absence of flexor muscles of the tarsus and the claw may be explained 
on the following grounds: the tarsus becomes defined as a segment distinct 
from the tibia only after the final molt, and is then practically immovable, 
while the claw in its normal resting position is bent over with its tip 
touching the ventral anterior extension of the tibia, so that only an extensor 
muscle is necessary for its function. No mechanism for ejecting the 
protractile disk has been found, and, as Osborn suggested, this ejection 
may be accomplished by means of an elastic framework. 
The foregoing account deals only with what may be called the skeletal 
muscles of the louse. The muscles controlling the various systems of 
the body are described later in their respective connections. 
The histological structure of the muscle is best seen in the material 
fixed in Bouin’s solution and stained with Mallory’s anilin-blue connective- 
tissue stain, when all the cross-striations stand out with great clearness. 
All the muscles have a well-developed sarcolemma zu are richly supplied 
with peripheral nuclei. 
THE VASCULAR SYSTEM 
In the writings of the early investigators of the Pediculidae, no real 
description of the dorsal vessel is to be found. Landois (1864:11), after 
many attempts, distinguished in freshly molted insects a slender tube 
originating in the region of the transverse tracheal band. He traced it 
cephalad to the middle of the abdomen, but could follow it no farther. 
Its pulsations were more rapid than those of the stomach. Mjéberg 
(1910:223) pointed out the similarity of the heart in the two groups 
which he studied, and drew attention to the lack of any thorough work 
in the Siphunculata (Anoplura). According to Schréder (1912-13:390), 
Provazek in 1905 described and figured the heart of Haematopinus 
spinulosus Burm., and this appears to be the first anatomical description 
of the heart of a siphunculatan. Miiller (1915:27) has figured the heart 
of the clothes louse in gross and in sections, and has described in detail 
its anatomy and its pulsations in living specimens. Harrison (1916 b:220) 
