136 THE VITAL FUNCTIONS. 



poses to which they are subservient in the process of diges- 

 tion, I pass them over as being foreign to the object of our 

 present inquiry.* 



It is essential to the perfect performance of digestion, that 

 every part of the food received into the stomach should be 

 acted upon by the gastric juice; for which purpose provi- 

 sion is made that each portion shall, in its turn, be placed 

 in contact with the inner surface of that organ. This is the 

 more necessary, as many facts render it probable, as will be 

 noticed more particularly hereafter, that, besides the chemi- 

 cal action of tjie gastric juice, an influence, derived from the 

 nerves, essentially contributes to the accomplishment of the 

 chemical changes which the food undergoes in the stomach. 

 For this reason it is that the coats of the stomach are pro- 

 vided with muscular fibres, passing, some in a longitudinal, 

 others in a transverse, or circular direction; while a third 

 set have an oblique, or even spiral course.t When the 

 greater number of these muscles act together, they exert a 

 considerable pressure upon the contents of the stomach; a 

 pressure which, no doubt, tends to assist the solvent action 

 of the gastric juice. When different portions act in succes- 

 sion, they propel the food from one part to another, and 

 thus promote the mixture of every portion with the gastric 

 juice. We often find that the middle transverse bands con- 

 tract more strongly than the rest, and continue contracted 

 for a considerable time. The object of this contraction, 

 which divides the stomach into two cavities, appears to be 

 to separate its contents into two portions, so that each may 

 be subjected to different processes; and, indeed, the differ- 

 ences in structure, which are often observable between these 

 two portions of the stomach, would lead to the belief that 

 their functions are in some respects different. 



* These structures have been examined with great care and minuteness 

 by Sir Everard Home, who has given the results of his inquiries in a series 

 of papers, read from time to time to the Royal Society, and published in 

 their Transactions. 



f See Fig-. 51, vol. i. p, 106, and its description, p. 107. 



