THYMUS 333 



situated on the ventral side of the larynx and trachea, and if well 

 developed, constituting a " middle lobe." 1 



The function of the thyroid is not thoroughly understood, but 

 the organ is essential for the well-being of the individual, its 

 extirpation commonly resulting in various disturbances of the 

 mental and organic functions. It is extremely vascular, especially 

 in Mammals, and gives rise to a substance which may contain 

 iodine. 



THYMUS. 



The thymus has always a paired, epithelial origin, and is thus 

 primarily of a glandular nature : its epithelial character is retained 

 throughout life, although the large cells of which it is orginally 

 composed undergo a marked subdivision into smaller elements. 



In Cyclostomes, the presence of a thymus has not been proved. 

 In Elasmobranchs it arises on either side from the endodermic 

 epithelium lining the upper angles of the first five gill-clefts, near 

 the ganglia of the ninth and tenth cerebral nerves, as well as in 

 the neighbourhood of the spiracle : it appears that all the gill- 

 pouches originally took part in its formation, as is also indicated 

 in Teleosts, Caecilians, and even in Snakes. 



In Teleostomes and Dipnoans, the thymus is similarly situated 

 dorsally to the branchial region, but certain modifications occur, 

 part of it undergoing resorption, while a subdivision into lobes or 

 a secondary fusion of originally distinct parts occurs. In adult 

 Urodeles and Anurans it lies behind and above the articulation of 

 the lower jaw. 



In all the Amniota, the thymus is developed in connection 

 with the three or four anterior pharyngeal pouches. In 

 adult Snakes, and also in Lizards and Chelonians, it consists of 

 two or more separate lobes situated near the carotid arteries. 

 Young Crocodiles and Birds possess an elongated, band-like, lobed 

 thymus extending along the neck (Fig. 243). 2 In Mammals 

 usually only a small portion of the organ is present in the cervical 

 region, its greater part being situated in the thorax, just above 

 the sternum. In young animals it is usually very voluminous, and 

 increases in size up to sexual maturity, then gradually becoming 

 reduced, without however, losing its function, whatever that 



1 Under the term parathyroids, or " accessory thyroids," are understood those 

 parts of the thyroid which may arise from its unpaired rudiment (ductus 

 thyreo(/lo**iis). The so-called " epithelial bodies" formed from the second to the 

 fifth pharyngeal pouches, as well as the ultimobranchial (postbranchial) body 

 arising from the most posterior cleft, have nothing to do with the thyroid, 

 although they may come into intimate relation with it as well as with the 

 thymus. The structure known as the "carotid gland" in Mammals, which is 

 situated at the bifurcation of the common carotid artery of either side, belongs 

 to a peculiar category of organs which have genetic relations to the sympathetic 

 system (cf. p. 247) : a similar structure is said to be present in Birds. 



2 Muscular elements occur in the thymus of Amphibia and Sauropsida. 



