tlii- is due to the recalling of previous sight and 

 smell LmpreMtoiui in tin- I. 



\\ IH-II food M passed into the Htoinach, secretion 



occurs. Thi- too may result from mechanical 

 irritation, a.s when through an o|M-niiig ( listula ) the 

 mucous memliram- i- luii-ln-d with a feather. In 

 all cases the Btonuu-li, pale In-fore, IM-COHH-.. -u!lu-ed 



\vitli II|MM|. ami 

 the gastric juice 



is poured (ut. 

 Tin- How of the 

 intestinal juice, 

 tin- pancreatic 

 juice, and liili-, 

 all follow the 

 .stimulation of 

 the in u c o u H 



membrane, ami 



in all cases the 

 blood-vessel- en 

 large HO as to 

 give the cells a 

 good supiily of 

 food, although, 

 as we have -n, 

 they them-elve- 

 actually pa.-s 

 into a condition 

 of activity, as a 

 M-ult of the in 

 II H. 'iice of special 

 secreting nerves. 

 We have now 

 to consider the 



Fig. 13. Nervous Mechanism of 

 Secretion : 



B, brain ; C, spinal cord ; S, nerve pass- 



ing from mucous membrane of mouth 



to brain ; 88, nerve passing to cell in 



salivary gland; VM, nerve passing to "i^'^t ion of food- 



blood- vessels of gland. stuns, and the 



pan- that the 



various glands play in this important process. For 

 our present purpose it is sufficient to speak of 

 foods as l>elonging to four classes: (1) Proteids 

 albumens, globulins, &c. e.g. the white of egg, 

 the chief constituent of meat, the gluten of bread. 

 (2) Carbohydrates starches, sugars, gums e.g. 

 potato-starch, cane or grape sugar. (3) Fats and 

 oils e.g. suet, marrow, olive-oil. (4) Minerals 

 e.g. water, table-salt, iron, phosphates. See also 

 DIET, FOOD. 



Some few substances are absorbed without l>eing 

 digested at all ; they do not need to be. Such are 

 water and the minerals, although even many ot" 

 these undergo some change or other. (Jrajn'-sugar 

 is absorbed, and probably proteids too are often 

 absorbed to some extent at least. Fat is profoundly 

 modified during digestion, although not as the 

 result of any digesting ferment. It mav I** .-aid. 

 however, that the great mass of food -stun" which is 

 classed under the head- of proteids and cariio- 

 hvdrates is all digested bv the action of ferment 

 Hike IxMlies. The saliva, of which altout 30 ounces 

 an- -ecreted during the '24 hour-, contains a ferment 

 termed ptyalin, which is capable of turning starch 

 into a soluble sugar called gra|M--siigar, or accord- 

 ing to other observers, into another soluble sugar 

 termed maltose. This can easily IK- dcmon-t rated 

 in the following manner: Take % tiny pinch of 

 ordinary starch powder, and shake it up with a 

 talile-poonful of wnt<T. It will not dissolve. Then 

 boil it, when the starch will swell up, formi 

 thin starch paste. Dilute a few drop- of tincture 

 of iodine with several volume.- of water, and add a 



drop of this to a small quantity of the starch 

 It will become of a rich blue colour. Tin- i- tin- 

 test for starch. Now add to some of the original 

 starch paste about one quarter of it- volume of 

 saliva from the mouth, and keep the niixtm 

 the fire at a temperature pleasant to the hand. In 

 a minute or so take out some of the starch in order 

 to test it again. The iodine no longer gives a blue 



colour, but produce* a beautiful mahogany brown. 

 Tin. -how* that tin- Mar.-h ha* disappeared. tl,- 

 brown colour U-in- the r.-u. t ion given by erytl.ro 

 dextrine into which tin- -lan-li ha* born converted. 

 a little time another t<orti<m tested in a 

 similar way give** no reaction w hat-v er, tin- -r> tlu. 

 di-xtrim- havm- in itH turn di-ap|--ar.-d Thentarrh 

 is now converted into HUgar, mixed with a little 

 unconverted dextrine. Tin- la-ing an arhroo- 

 dexirine. uneolourud like the <-r\ thru d<-\Uiin- by 

 tincture of iodine, 



ii Water. Orp-M0tf. 



r,H to 4 * H,0 -- r.H,/) r 



The ptyalin mav be extracted from the naliva or 

 from the salivary glands themselves. It i\>- not 

 appear to )>e much exhausted during it- arti\i(>. 

 and has the general characters already disctwwed 

 of unorganised ferment-. The saliva U alkaline, 

 and the starchy food is no doubt partly converted 

 into sugar during its sojourn in the mouth and 

 gullet by its action. When tin- f.M,d has n-a. -h.-d 

 the stomach and the acid gastric juice ha* mixed 

 with it, the saliva is unable to act, and is pmliahly 

 killed. Any undigested starch is sul>-e|uentlv 

 converted into sugar when the f<xd reaches the 

 small intestine by the pancreatic juice. 



Wh?n the food reaches the stomach it Cannes 

 a reflex secretion of gastric juice. This i- but 

 slowly produced when insipid heavy food. Mich as 

 coagulated white of egg, Itoilcd meat. -.._. \c., in 

 eaten, but (lows readily when soup-, broths, and 

 fluid- containing >alt- and extractions in ahund- 

 anqe are taken. Thus we have a scientific reavon 

 for commencing a dinner with soup, preserving the 

 pitce de rttutancc until the stomacn lui> secreted 

 gastric juice for its digestion. The gastric juice, 

 several pounds of which are secret-d daily, is acid 

 in reaction containing free hydrochloric acid. In 

 addition lactic and butyric acids are formed during 

 the progress of digestion. The ferment |>eculiar to 

 the stomach is termed pe]*ine, and it require*, 

 unlike the ptyalin of the saliva, an acid medium 

 for it -act ion. I'ep-ineand hydrochloric acid conv. rt 

 jtroteids into -iil-taiices termed jH-ptom-. The latter 

 iMxiies are soluble in water, ami are not coagulated 

 by lioiling like many of the protei.l-. In addition 

 they pass readily tnrough an animal membrane. 

 and are therefore easily alorlied. The prot-id 

 does not pa. directly into the p-ptme. but like the. 

 starch is changei into at lea-t one intennediate 

 IMM!V. In the h'ret case the proteid bcomies ...n 

 I verted by the hydrochloric acid into whnt i* called 

 -\ntonin. or acid protein, and it it* this syntonin 

 which alone the pep-ine has jMiwer to change into 

 jH'ptone. The -yntonin Ix-come- a pn>|H-|itone 

 which is intermediate in it- piojM-rtie- U-tMcfn a 

 proteid 'Hid in-ptone itself. It i- vrr\ probable that 

 the whole change fiom j>rtteid to peptone iinfa4* 

 in -ome -imple change in the iiiolit-iilar chni 

 of the proteid. together with the addition ol water. 

 A peptone ma\ U- termed therefore a hydrated 

 proteid. and the jK-p-ine a hvdmlytic fennent. 

 Within the stoma-h, ami ca|>a)>le of U-ing readily 

 extractel from the -tomach of calf, is n fermrnt 

 called the milk coagulating ferment (rennet ). Thi* 

 causes milk t> coagulate, and form a clot or curd. 

 It -ul.-e.jut-ntlv c..ii!i.t, i- and aqaeen* oat MOM 

 fluid termed the whry. It i- diHicult to amign any 

 u-e to thi- ferment, <W the acid of the gn- 

 would CHUM* of its'lf the milk to curdle. Still 

 more ditticult is it to explain the existence of 

 a similar ferment in the pancrvatic juice, for 

 we cannot -up|Mte that any milk can even pa** 

 through the stomach to be subjected to it* action. 

 The milk unbitl oft.-n forms dense dot* which 

 cannot afterwards be digested, and which cause 

 considerable gastric irritation. Milk when drunk is 



