L. E. Eohinson and J. Davidson 
231 
If, now, the form of the pharynx is considered, it is seen that it is 
such as to allow a great range of variation in the capacity of the organ, 
accordingly as its walls are in contact or separated. Separation of the 
pharyngeal walls, and consequently dilation of the pharyngeal cavity, is 
brought about by the larger radially-disposed muscle bands—the dilator 
muscles of the pharynx—and by tbe contraction of the smaller circularly- 
disposed muscles—the constrictor muscles of the j^harynx-—the pharyn¬ 
geal walls are brought into apposition and the internal cavity is 
practically obliterated (see Text-fig. 8). 
The function of the pharynx being obvious, it now becomes necessary 
to see what provision is made to ensure that the ingested blood is forced 
in one direction, viz. towards the stomach. We have made repeated 
Fig. 3. Diagrammatic representation of the cross section of the pharjmx :—A, in a state 
of contraction; B, in a state of dilation. The action of the muscles is indicated 
by the arrows. 
efforts to discover a valvular structure at the anterior pharyngeal orifice, 
but without success. The lumen of the organ, as already stated, is 
constricted at this point, but nothing of the nature of a valve can be 
recognised. It would, therefore, appear that the vis-a-tergo of the blood 
in the buccal canal and buccal cavity is greater than that of the contents 
of the oesophagus and stomach, so that when the pharynx contracts, its 
contents follow the direction of least resistance, i.e. through the oeso¬ 
phageal orifice. Having reached the stomach, regurgitation of the 
ingested blood is prevented by the peculiar structure of the proximal 
end of the oesophagus. As will be shown later, the oesophagus is 
telescoped for a short distance into the cavity of the stomach, forming 
