PHARYNGEAL DERIVATIVES 



263 



clefts I and 2, while in the amniotes (fig. 283, C) clefts 3 and 4 are the only 

 sources of thymus material. 



These ingrowths result in a richly vascular organ which, by the 

 intrusion of connective tissue, may assume an acinous form. Its 

 form and position vary rather widely in the various groups of 

 vertebrates. 



In Myxine a number of lobules behind the gill region have been 

 interpreted as a thymus by some, by others as pronephric in origin. 

 In the fishes the thymus is an elongate, much lobulated organ behind 

 the dorsal ends of the gill arches (usually the fourth arch in teleosts). 

 In most amphibia it is small and lies behind the angle of the jaw and 

 the tympanic cavity, but in the cajcilians (and snakes) it consists of a 

 series of elements corresponding to the clefts in position, the original 



Fig. 284. — Phar>'ngeal derivatives in (A) lizard, (S) hen, and (C) calf, after de 

 Meuron. c, carotid artery; j, jugular vein; h, heart; p, parathyreoid; sc, subclavian 

 artery; /, trachea; tm, thymus; tr, thyreoid. 



branchiomeric character being retained. In the reptiles it is in the 

 sides of the neck (fig. 284,^), where it often consists of two or three 

 sections. In the birds it is more elongate and band-like (fig. 284, B). 

 In the mammals it is at first in the sides of the neck (fig. 284, C), but 

 in the adult it has been carried back into the thorax, from which it 

 more or less extends forward. The thymus glands of several domes- 

 tic animals are sold in the markets as 'throat sweetbread.' With 

 increase in age, the gland in mammals (the whales and seals excepted) 

 tends to grow (in man) until about the fifteenth year, after which 

 it becomes smaller. It was formerly said to reach its maximum at 

 the second year, and then gradually to atrophy by conversion into 

 fatty tissue, but it retains its functional structure until middle life. 

 Closely related to the thymus glands in origin, but not in func- 



