DEVELOPMENT OF THE PHARYNX 2 19 



The first recognition of the significance of the mammalian gill clefts is credited to 

 Rathke, in 1832, who published the following significant conclusions in his "Unter- 

 suchungen iiber den Kiemenapparat der Wirbelthiere." 



"In all vertebrates without exception, in the earliest period of development, there 

 are formed the beginnings of a branchial apparatus. Its elements vary in number in 

 the different vertebrates, yet in tissue, form, position and connections they are very 

 similar to one another, and are built upon the same plan. Their development, how- 

 ever, proceeds along different lines in the various animals. In some it is partly re- 

 gressive, bringing about the most manifold and divergent modifications of these 

 structures, not merely in form but also in tissue, type, and significance. Yet there 

 always remains an analogy between them; and through easy transitions, the forms and 

 types pass into one another from the bony fishes even to man. The branchial appara- 

 tus is most highly developed in fishes; in the other vertebrates its development is the 

 less complete, the further, in general, these vertebrates are removed from the fishes." 



The mammalian gill clefts, although rudimentary as branchial organs, 

 are of the utmost anatomical importance. A single large artery passes 

 from the ventral aorta to the dorsal aorta between the successive pouches, 

 and also in front of the first and behind the last. These aortic arches 

 therefore number one more than the series of pouches; from them, portions 

 of the aorta, carotid and subclavian arteries are produced, as described 

 in works on embryology. The nerves send trunks down between the 

 pouches, the facial nerve being between the first and second, the glosso- 

 pharyngeus between the second and third, and the superior laryngeal 

 branch of the vagus between the third and fourth. Thus these structures 

 determine the arrangement of the vessels and nerves. 



On the basis of comparative studies the presence of a fifth pouch in mammals was 

 predicted, and the posterior arm of the Y-shaped outgrowth, including the postbran- 

 chial body, is often described as such. A branch of the superior laryngeal nerve is said 

 to pass between the arms of the Y, but a typical branchial relation between the nerves 

 and the fifth pouch has not as yet been established. A "fifth aortic arch" is often rep- 

 resented as passing between the fourth pouch and the postbranchial body, but it has 

 been shown that this arch differs from all the others in its order of development (form- 

 ing only after the "sixth" is complete). Whereas the third, fourth, and last aortic 

 arches all produce very important vessels, the questionable "fifth arch" is an insignifi- 

 cant plexiform anastomosis, which disappears rapidly. Small vessels, however, are 

 always to be found near the postbranchial body in rabbit, pig and human embryos 

 measuring 5-10 mm. The most convincing evidence of the presence of a fifth pouch is 

 an actual contact with the ectoderm, posterior to the fourth pouch; this was recorded 

 by Hammar in a s-mm. embryo, but the contact on either side took place in only one 

 12 fi section. Grosser states that a closing membrane "is perhaps not always formed, 

 and is at all events very transitory" (Human Embryology, ed. by Keibel and Mall, 

 1912, vol. 2). There are as yet very few observations to show that it ever occurs in 

 mammalian embryos. The existence of a sixth pouch has been asserted on the basis 

 of slight elevations which are perhaps inconstant. 



TONSILS. 



The palatine tonsils are two rounded masses of lymphoid tissue, one 

 on either side of the throat, between the arches of the palate (Fig. 207.) 



