ALIMENT AR Y S YSTEM. 453 



some cases their openings are very evanescent. The clefts 

 are bordered by the branchial arches, and supplied by blood 

 vessels and nerves, 



With the anterior part of the alimentary canal two 

 strange structures are associated — the thyroid and the 

 thymus. 



The thyroid gland arises as a diverticulum from the ventral wall of 

 the pharynx. It may be single (as in some Mammals), or bilobed (as 

 in Birds), or double (as in some Mammals and Amphibians), or diffuse 

 (as in Bony Fishes). Only in the larval lamprey does it retain its 

 original connection with the pharynx, and is then a true gland. 



As to its morphological nature, its mode of origin suggests com- 

 parison with the hypobranchial groove in Amphioxus and the endostyle 

 of Ascidians. According to Dohrn, it is a residue of the visceral cleft 

 between the hyo-mandibular and the hyoid. It sometimes has accessoiy 

 parts derived from a visceral cleft (fourth in Mammals). 



Almost the only light which has been cast on the physiological nature 

 of the thyroid is from the pathological side. Goitre and Derbyshire 

 neck are associated with an enlargement and diseased state of this organ, 

 and myxcedenia 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, perhaps through some specific ferment. 



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 

 rudiments. In Mammals it often seems to degenerate after youth. As 

 it has from its first origin a distinct lymphoid nature, and apparently 

 forms leucocytes, it has been interpreted (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 digestive, 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 Vertebrates, grow out. The 

 air-bladder usually lies dorsally and is almost always single ; 



