THE VISCERAL ARCHES. 



6i 



The Third or First Branchial Arch contains a rudimentary cartilaginous 

 bar from which part of the body and the greater cornu of the hyoid bone are derived 

 The fourth and fifth arches, or second and third branchial, enclose rudimentary 

 cartilaginous bars which early fuse into plates ; these unite along their ventral borders 

 and give rise to the thyroid cartilage of the larynx. 



The External Visceral Furrows (Fig. 73), the representatives of the true 

 clefts of the lower types, appear with decreasing distinctness from the first towards the 

 fourth ; the third and fourth early suffer modification, so that by the twenty-eighth 

 day the first and second furrows alone are clearly defined. 



The First Visceral Furrow, the hyomandibiilar cl^ft, undergoes obliteration 

 except at its dorsal part, which becomes converted into the external auditory meatus, 

 the surrounding tissue giving rise to the walls of the canal and the external ear. The 

 remaining clefts gradually disappear, becoming closed and covered in by the over- 

 hanging corresponding arches ; this relation is particularly marked towards the 



A 



Fig. 73. 



Optic vesicle 



Maxillary process — f 



Dorsal wall of primitive 

 oropharynx 



Primitive CESophagus 

 Upper end of body-cavity 



Right umbilical vein 



Primitive larynx 

 IV 



O iV.- Body-cavity 



Primitive aortse -• 

 Neural tube 



Upper half of human embryo of about eighteen days, drawn from His's models. X 45. Ay dorsal wall of primitive 

 oropharynx bounded by visceral arches, external and internal furrows. B, anterior wall of primitive oropharynx, 

 seen from behind. 1-5, sections of aortic arches ; I-IV, external visceral furrows. 



caudal end of the series, where the sinking in of the arches and the included furrows 

 produces a depression or fossa — the simis prcscervicalis of His — in the lower and lateral 

 part of the future neck region. This recess subsequendy entirely disappears on 

 coalescence of the bordering parts ; sometimes, however, such union is defective, 

 the imperfect closure resulting in a permanent fissure situated at the side of the neck, 

 known as cervical fishila, by means of which communication is often established 

 between the pharynx and the exterior of the body. Such communication must, 

 however, be regarded as secondary, as originally the external furrows were sepa- 

 rated from the primitive pharyngeal cavity by the delicate epithelial septum already 

 mentioned as the occluding plate. Where entrance into the pharynx through the 

 fistula is possible, it is probable that the septum has been destroyed as the result 

 of absorption or of mechanical disturbance following the use of the probe. 



The Inner Visceral Furrows, or pharyngeal ponches, repeat the general 

 arrangement of the external furrows. The first pharyngeal pouch becomes 

 narrowed and elongated, and eventually forms the Eustachian tube • a secondary 



