450 STRUCTURE OF VERTEBRATA. 



Sauropsida and Mammals, in which there are no gills, four 

 "visceral" clefts persist as practically functionless residual 

 structures. In some cases their openings are very evane- 

 scent. 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 Mam- 

 mals), 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 con- 

 nection with the pharynx, and is then a true gland. 



As to its morphological nature, its mode of origin suggests 

 comparison with the hypobranchial groove in Amphioxus 

 and the endostyle of Ascidians. According to Dohrn it is a 

 residue of the gill cleft between the hyo-mandibular and the 

 hyoid. 



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 myxcedema with its degeneration or absence. As injection 

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

 dema, it is concluded that the thyroid must naturally 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 five in the skate, four in Tele- 

 osteans, three in the lizard, one in the chick ; 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 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 digestive, absorptive, conducting 

 intestine, ending in the rectum and anus. 



From the oesophagus, the air- or swim-bladder of most 



