174 MR. FE. G. PARSONS ON THE [Jan. 14, 
surface of the pouch, and runs backward and upward under the 
ear to be lost on the back of the neck. When this is dissected 
away, there is seen to be a deeper layer of muscle having exactly 
the same direction, and also coming from the surface of the pouch. 
By far the larger portion of the outer wall of the pouch is formed 
by the sterno-facialis muscle, which is especially thick at the 
orifice forming a partial sphincter; the fibres of this muscle run 
downward and backward to be inserted into the anterior half of 
the sternum, superficial to the pectoralis major. There is no 
panniculus in the inner wall of the pouch, which consists solely of 
skin covering the masseter and buccinator muscles, with the 
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Pouch-muscles of Heteromys. 
exception of one narrow slip which rises from the mental 
symphysis, runs round the inner side of the opening of the pouch, 
and when it reaches the lower margin turns sharply outward, 
superficial to the fibres of the sterno-facialis, to run to the side of 
the neck, where it joins the platysma. This decussation of the 
fibres guarding the onter and the inner side of the aperture 
gives a powerful sphincteric action by which the pouch can 
be closed at will. It is worth while to compare the anatomy of the 
pouch in the Geomyide with that in Cricetus, and to notice that, 
whereas in the former the pouch is formed by an invagination of 
skin probably perforating the platysma, so as to leave some of 
