102 DEVELOPMENT OF THE TONGUE. 



appearance ; figs. 117, 118, seen from within). 1 Between them, and also in front of 

 the first cleft, the phaiyngeal wall is greatly thickened so as to exhibit the appear- 

 ance of curved bars bounding the clefts ; these bars are known as the cephalic 

 visceral arches, and are five in number, viz. : the first or mandibular, in front of the 

 first visceral cleft, between it and the mouth : this is the seat of formation of the 

 lower jaw ; the second or htjoid arch, between the first and second clefts ; the third 

 or thyro-hyoid arch, between the second and third clefts, which in fishes and 

 amphibia develops gill-plates, and is therefore also known as the first branchial 

 arch ; the fourth between the third and fourth clefts, corresponding with the second 

 branchial arch of fishes, but small and inconspicuous in man and mammals ; and the 

 fifth or second branchial still smaller and more inconspicuous, forming the posterior 

 boundary of the fourth cleft, and hardly recognizable as a distinct bar in man. 

 After the fourth week, and with the increasing flexure of the head, the arches become 

 somewhat shifted over one another, so that the fourth arch is concealed by the third, 

 and the third by the second. 



The mandibular arches early become united on the ventral aspect ; from the fifth week 

 their union is complete (fig. Ill, /.) and in man shows eventually no sign of a median groove. 

 The other arches do not at first reach the middle line (His), the space between their central 

 ends being occupied by the heart and pericardium ; as these shift backwards a smooth infra- 

 mandibular surface is left externally. 



Development of the tongue. Within the pharynx, the second and third 

 arches of the two sides are separated by a forked elevation (furcula) with a median 



Fig. 120. POSTERIOR ASPECT OP THE VISCERAL ARCHES OF THE EMBRYO 



SHOWN IN FIGS. 116, 117, AS SEEN FROM THE INTERIOR OF THE 

 PHARYNX. (His. ) ** 



The first or maudibular pair of arches join in the middle line ; the 

 second arches are separated by a rounded prominence (tuberculum 

 impar). Behind (below) this is the forked prominence (furcula) 

 bounding a median groove which will become the laryngeal orifice. In 

 the sections of each of the first two arches, the included artery is seen. 

 The Roman numerals are opposite the corresponding arches. 



groove, in front of which is a rounded tubercle (t. impar, 

 His), which arises in the angular space between the 

 first and second arches (fig. 120). The groove around 

 the furcula (sinus arena fas f His) passes laterally into 

 the visceral clefts. The second and third arches afterwards unite between the 

 furcula and tuberculum impar (fig. 121, A). Thus united, the junction forms an 

 X-shaped mass. 



From these conjoined extremities of the second and third arches on either side, the 

 root of the tongue grows upwards and forwards as two prominences, which diverge 

 in a \/-shaped manner to embrace the anterior or papillary part of the organ which 

 is developed from the tuberculum impar (fig. 121, B). At the angle of the V is a 

 deep depression (foramen ccecum) ; this leads into a diverticulum, which forms the 

 median rudiment of the thyroid body. When the parts of the tongue are united, 

 there is still for a considerable time a V" sna P e( i groove marking the line of union 

 (fig. 122), and even in the adult there is often a distinct trace of this groove (sulcus 

 ferminalis, His). Parallel to this, and somewhat in front of it, the papillae vallatas 

 are developed, and in front of these the other lingual papillae make their appear- 

 ance (about the end of the second month). 



1 According to His, who is confirmed by Born and by Kolliker. these clefts are not as a rule developed 

 into complete apertures in birds or mammals ; although the membranes which close them are composed 

 only of juxtaposed epi- and hypoblast, the inesoblast having disappeared. 



