174 iin. r. a. paesons on tub [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 bo 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-faeialis 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 



Fig. 8. 



-Stei\.-fac. 

 platvsma. 



Pouch-musclea of Hetaromys. 



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-faeialis, to run to the side of 

 the neck, where it joins the platysma. This decussation of the 

 fibres guarding the outer and the inner side of the aperture 

 gives a powerful sphinctoric action by which the pouch can 

 be closed at will. It is worth while to compare the anatomy of the 

 pouch in tlie Geomyidaj 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 



