1901.] NATURAL SCIENCES OP PHILADELPHIA. 241 



I will now return to the " diverticulum " in order to trace its 

 further course. From its distal extremity the " diverticulum " 

 extends inward and slightly backward in close contact with the 

 anterior surface of the depressor ossis hyoidei (vi.cl.h., fig. 24, 

 Tym.), and then ascending slightly to pass over a large vessel, the 

 mandibular aortic arch (m.a.), it comes into close relation with 

 the external surface of the processus muscularis of the quadrate. 

 As it progresses inward the cord gradually decreases in diameter, so 

 that when it reaches the quadrate it is reduced to about a half or 

 even a third of the diameter of its distal expanded portion. 



After reaching the external surface of the quadrate the reduced 

 ' ' diverticulum ' ' turns sharply posteriorly at the inner edge of the 

 depressor ossis hyoidei as a minute, cylindrical cord, (ties. 23, 15, 

 19, Eu.). Here it is closely applied to the processus muscularis 

 of the quadrate. Below and internal to it is the mandibular 

 aortic arch (m. a.), while bounding it externally is a small, acces- 

 sory slip from the depressor mandibulie (fig. 15, vi.d.m'.), the main 

 body of which is attached to the quadrate anterior and internal to 

 the cord (fig. 24, m.cl.m.). The cord extends posteriorly in the 

 same position, usually closely applied to the quadrate, and showing 

 more or less reduction in size, so that in certain parts of its course 

 it is difficult to trace clearly. Throughout its entire extent, how- 

 ever, it contains numerous pigment granules, the presence of 

 which facilitates considerably the tracing of the cord, as does also 

 the scattered yolk-bodies apparent for the last time in the present 

 stage. 



In fig. 15 the cord can be seen, much reduced, just under the 

 transversely extended processus muscularis and external to the 

 mandibular aortic arch {m.a.). In fig. 19 (right side) the cord 

 (Eu.) still occupies the same relative position. Just external to it 

 is the ramus hyomandibularis of the facial nerve (vii h.). Inter- 

 nal to it the mandibular artery (m.a.) intervenes between it and 

 the pharyngeal wall. At this point the mandibular aortic arch 

 begins to turn inward in order to reach the carotid. Immediately 

 behind the artery the cord fuses with the distal extremity of the 

 diverticulum extending up from the pharyngeal wall (see fig. 19, 

 Eu., left sixle). Here both the cord and proximal portion of the 

 hyomandibular fold become continuous. The fold becomes more 

 prominent in the following sections (fig. 23, Ea.) and ultimately 

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