256 



COMPARATIVE ANATOMY 



and left lobes lying on the great vessels just after they leave the 

 heart. In Birds (Fig. 205) the organ is paired, and lies close to 

 the origin of the carotid arteries. 



The thyroid of Mammals consists of two lobes often connected 



by a median' isthmus, situated on 

 the ventral side of the larynx and 

 trachea (Fig. 190). 



It appears probable that the 

 thyroid represents a very ancient 

 glandular organ, the secretory func- 

 tion of which in relation to the 

 alimentary canal was of great im- 

 portance in the ancestors of Verte- 

 brates. In existing forms it has 

 undergone a change of function, and 

 thus instead of disappearing, remains 

 as an important organ in the adult : 

 in Mammals especially it is charac- 

 terised by a great richness in blood- 

 vessels. What this function is, is not 

 thoroughly understood, but it has 

 been shown that its albuminous 

 secretion contains iodine, and is 

 passed into the blood- and lymph- 

 vessels ; and that extirpation of the 

 organ is followed by various distur- 

 bances of the mental and organic 

 functions. 



The structure known as the ' ' carotid 

 gland " in Mammals, which is situated at 

 the bifurcation of the common carotid into 

 external and internal carotids, has not, as 

 was formerly supposed, anything to do with 

 the thyroid or thymus. It is abundantly 

 provided with nerve-cells. 



Thymus. 



The thymus has always a paired 

 origin, and iu the adult consists of 

 lymphoid tissue. In Elasmobranchs 

 it arises on either side from the epi- 

 thelium lining the upper part of 

 the first five gill-clefts, close to the 

 ganglia of the ninth and tenth cere- 

 bral nerves, as well as in the neigh- 

 bourhood of the spiracle. The func- 

 tion of this organ, though doubtless 

 avery important one, is not understood. 



Fig. 205. — Thymus and Tiivimud 

 or A YouNO Stokk. 



T, trachea ; B, bronchi ; Oe, oeso- 

 phagus ; H, heart ; Tm, thymus ; 

 Tr, thyroid. 



