578 CLASS xvn. 



The cavity, which forms the commencement of the alimentary 

 canal, the gullet (pharynx), is wider above and passes below into 

 the narrower oesophagus; it is formed by a continuation of the in- 

 teguments of the nasal cavity and of the mouth, and is surrounded 

 by many muscles which are inserted into different parts of the 

 tongue-bone, into the cartilages of the larynx, and into the sphenoid 

 and temporal bones. In the true cetaceans the larynx ascends high 

 up in the pharynx to the nasal cavity, so that the food descends in 

 deglutition on each side of the larynx, and water is thus prevented 

 from entering the windpipe 1 . 



The oesophagus is a nearly cylindrical tube, which runs through 

 the cavity of the thorax, and lies towards the vertebral column ; 

 usually it is long, and forms the narrowest part of the intestinal 

 canal. The muscular coat consists of two layers of fibres; the 

 external layer is in man composed of longitudinal, the internal of 

 transverse circular fibres. In most mammals the fibres of the two 

 layers are turned spirally and run in two opposite directions. The 

 mucous membrane and the innermost covering of epithelium form 

 longitudinal folds which become more conspicuous on contraction 

 of the muscular coat. The oesophagus of some mammals has in 

 addition at the inferior extremity transverse, circular folds ; they may 

 be observed in the tiger, the lion, and other carnivorous animals 2 . 



The oesophagus when it has perforated the diaphragm passes, 

 sometimes after continuing its course for a space in the abdominal 

 cavity but in most mammals almost immediately below the dia- 

 phragm, into the stomach. In most mammals the stomach is sim- 

 ple, as in man. Often the cardiac portion is divided by a constric- 

 tion from the pyloric portion. In many rodents this constriction is 

 very conspicuous ; there is an accumulation of glands at the cardia 

 of the stomach, and in some species of the genus Myoxus this 

 glandular part forms essentially a peculiar dilatation of the stomach 3 , 

 a disposition which corresponds with the structure of the stomach 

 in birds (see above, p. 336). 



1 See a figure in CAMPER Cetac. PL L; and another in CAEUS Tabula Anat. comp. 

 illustr. Pars IV. 1835, Tab - vn - % IV - 



2 See HOME Lect. on Comp. Anat. Tab. XI. In the horse is a spiral valve which 

 makes a turn and a half. See a figure by GURLT in MECKEL'S Archivf. d. Physiol. vi. 

 Taf. 4, fig. 8. 



3 HOME 1. 1. Tab. xm. 



