Ill] THE DIGESTIVE ORGANS 33 



(depending upon two sets of nerve fibres) is proved by injecting 

 atropin, when stimulation of the nerve still prodiices all the 

 vascular changes, but no saliva is secreted, tlie atropin having 

 paralysed the secreting fibres without affecting the vaso-dilator 

 fibres. The existence of a secretory process occurring indepen- 

 dently of the vaso-dilator mechanism is further proved by the 

 fact that the j^ressure of the saliva in the duct at the time of 

 secretion may be greater than the blood pressure in the carotid 

 artery, and consequentlj' considerably greater than that in the 

 capillaries of the gland. Moreover secretion may take place in 

 the absence of blood, since after cutting off the head of a rabbit 

 and immediately stimulating the chorda, salivary secretion can 

 be induced. In the sympathetic two sets of fibres can also be 

 demonstrated experimental^, these being secretory' and vaso- 

 constrictor, but the secretory fibres are comp.aratively few. 



Further evidence of the existence of a definite secretory 

 mechanism is afforded by the presence of mucigen granules, and 

 the changes which take place during activity as described above, 

 as well as by the fact that mucin and ptyalin do not exist as 

 such in the blood, and therefore must l)e manufactured by the 

 glands. 



The Stomach, 



The alimentary canal throughout its entire length is sur- 

 rounded by several layers of tissue which have an essential 

 similarity throughout its entire length. In those parts which 

 lie freely in the body cavity there is an external serous or peri- 

 toneal coat consisting of fibrous tissue, and contiiuious with the 

 mesentery or fibrous sheet which slings the gut in position and 

 connects it with the inside of the body wall. Within the peri- 

 toneal coat are unstriated muscles arranged longitudinally and 

 circularly, and within the muscular coat is connective tissue 

 lined internally by a further thui muscular layer. The cavity 

 of the alimentary canal is lined by an epithelium which varies 

 in character in its different parts. In the mouth, pharjmx and 

 oesophagus, and in some cases the first part of the stomach, it is 

 stratified and in function mainly protective, but in the secretory 

 part of the stomach and in part of the intestine, the epithelium 

 is columnar and depressed into tubular glands. 



The stomach may be described as the dilated portion of the 



M. p. 3 



