500 



STRUCTURE OF VERTEBRATA. 



organ, and myxoedema with its degeneration or absence. As injection 

 of extract of sheep's thyroid, or even eating this organ, alleviates myx- 

 cedema, it is concluded that the thyroid must have some specific effect 

 on the large quantity of blood which flows through it. It is probably 

 safe to say that the thyroid aids in keeping the blood at a certain 

 standard of health, through some specific secretion. 



The thymus arises as a dorsal endodermic thickening where the 

 outgrowths which form the gill -clefts meet the ectoderm. It may 

 be associated with a variable number of clefts seven in the shark 

 Heptanchus, five in the skate, four in Teleosteans, three in the lizard, 

 one in the chick, and one (the third) in Mammals. In the young 

 lamprey there are said to be no fewer than twenty-eight thymus rudi- 

 ments. In Mammals it often seems to degenerate after youth. In the 

 rabbit it has its maximum weight in the fourth month, and thereafter 

 begins to be rapidly reduced. As it has from its first origin a distinct 

 lymphoid nature, and apparently forms leucocytes, it has been inter- 

 preted (Beard) as a structure adapted for the phagocytic protection of 

 the gills from bacteria, parasites, and the effects of injury. If this be 

 so, we can understand its diminishing importance in Sauropsida and 

 Mammalia, where its place may be to some extent taken by the palatal 

 and pharyngeal tonsils, which are believed by some (Stohr, Killian, 

 Gulland) to have a similar phagocytic function. 



The pharynx leads into the gullet or oesophagus, which is 

 a conducting tube, and this into the digestive stomach, 



which is followed by the diges- 

 tive, absorptive, conducting 

 intestine, ending in the rectum 

 and anus. 



From the oesophagus the air- 

 or swim- bladder of most Fishes, 

 and the lungs of higher Verte- 

 brates, grow out. The air- 

 bladder usually lies dorsally and 

 is almost always single; the 

 lungs lie ventrally and are 

 double, though connected with 

 the gullet by a single tube. 



The beginning of the intes- 

 tine gives origin to the liver, 



FIG. 266. -Origin of lungs, which reflates the composition 

 liver, and pancreas in the ot the blood and secretes bile, 

 chick. After Goette. and to the pancreas, which 



The mesoderm is shaded; the endo- SCCretCS digestive juices. The 



pancreas has often a multiple 



derm dark. 



f., One of the lungs ; St., stomach ; 

 /., liver ; /., pancreas. 



rudiment. 



