500 
MR. OSLER’S OBSERVATIONS ON THE ANATOMY 
which they are united, forming for it a sort of sphincter. There is a minute 
muscle between f, f, which, to prevent confusion, I have not figured along its 
whole course ; it is inserted under the lingual papilla, which it retracts. The 
transverse muscle g, and the oblique ones seen between the jaws, are portions 
of the great retractor of the tongue. 
To display this muscle, the upper jaw with its muscles, and the walls of the 
cavity of the mouth, are to be removed ; and the muscles f, f detached, and 
thrown forward. Having divided and turned aside the transverse fibres, g ; 
separated the ligamentous attachments which secure the lower edges of the 
accessory cartilages ; and cut the muscles k, k which throw the cartilages 
forward, we may slip out the jaws, and display the muscle as in fig. 12. It 
is composed of three distinct portions ; — the transverse fibres g which embrace 
and compress the lateral jaws ; a straight column on either side h , attached 
along the under surface of the lingual membrane ; and the oblique portions i, i, 
which, like the retractor muscles in Trochus, pass around the posterior carti- 
lages, and run forward to be inserted into the tongue itself. The only attach- 
ment of this muscle to the jaws being at the extreme points of the posterior 
cartilages, it is enabled to play over them with the greatest freedom. 
Chiton appears to feed like Patella, but there is considerable modification 
in the structure of the parts. A pair of simple lateral jaws, rather membranous 
than cartilaginous, constitute the whole skeleton. The tongue is projected 
around the point of these jaws by a pair of muscles corresponding to f,f, 
fig. 1 1. and 12 ; and is retracted by three pairs of powerful muscles ; of which 
two agree with h, h and i, i, fig. 12 ; while the third, arising from the tendon 
of the second valve of the shell, is inserted into the upper part of the tongue. 
Between the insertions of the last pair is the opening of the pharynx. The 
tongue is set on either side with two rows of large teeth, of which the inner 
present the form of circular discs, with very blunt edges ; the outer, corre- 
sponding to the interstices of the first, are prominent and falciform, with the 
points directed inward. The space between the inner rows is armed with 
ranges of smaller teeth. Under the opening of the pharynx, the tongue enters 
a sheath, in which its opposite edges are closely folded together. It conse- 
quently expands as it passes over the point of the jaws, and closes when re- 
tracted. Occupying the centre of the mouth is a large solid papilla, with an 
