COMPARATIVE PHYSIOLOGY OF DIGESTION. 343 



3. Diarrhea may develop in consequence of disturbances of the processes 

 of diffusion through the intestinal walls. Affections of the epithelial cells should 

 be mentioned in this connection: swelling in association with catarrhal or in- 

 flammatory conditions of the mucous membrane. As, further, in the process 

 of absorption independent activity on the part of the cylindrical cells is to be taken 

 into consideration, controlled, perhaps, by the nervous system, it is plain how 

 sudden agitation, from fright, anxiety, etc., may cause diarrhea. 



4. Diarrhea may be the result of increased secretion. In its simplest form 

 this occurs through capillary transudation, when salts, as, for example, magne- 

 sium sulphate, introduced into the intestine, remove water from the blood by 

 endosmosis. In this category belong the copious watery discharges that take 

 place in consequence of alteration of the intestinal epithelium, as in cases of 

 cholera, in which such excessive transudation takes place into the intestine that 

 the blood becomes inspissated and may even stagnate in the veins. In 

 addition, transudation into the bowel may take place in consequence of paralysis 

 of the yasomotor nerves of the intestine. The diarrhea due to cold appears to 

 belong in this group. Certain substances appear directly to irritate the secretory 

 organs of the intestine or their nerves ; among these are the drastic purgatives. 

 Pilocarpin injected into the blood also induces marked secretion. 



In the presence of febrile disorders, the secretion of the intestinal glands 

 appears to undergo quantitative and qualitative changes, with simultaneous 

 derangement in the activity of the intestinal musculature and the organs of 

 absorption and increased irritability of the mucous membrane. 



With respect to fermentations in the intestine, the fact should be emphasized 

 that all, in excess, as, for example, the butyric or the acetic, give rise to patho- 

 logical manifestation. With regard to the pathogenic schizomycetes acting 

 from the intestinal canal (cholera, typhoid, dysentery, and others) reference may 

 be made to p. 246 Flagellated trichomonads are exceedingly rare. 



Finally, attention should be directed to the fact that, in consequence of 

 abnormal decompositions in the intestinal canal, substances may be formed that 

 exert a toxic effect upon the organism and thus give rise to auto-intoxications. 



COMPARATIVE PHYSIOLOGY OF DIGESTION. 



Among mammals, herbivora possess larger salivary glands than carnivora, 

 while omnivora occupy an intermediate position. Whales have no salivary 

 glands at all; the pinnipeds have a small parotid, the echidna none at all. The 

 dog, like some carnivora, has an additional zygomatic gland situated in the orbit. 

 In birds the salivary glands empty at the angle of the mouth; the parotid 

 gland is wanting. Among snakes the parotid glands are in some species 

 transformed into poison-glands; tortoises have sublingual glands; in addition, 

 reptiles have labial glands at the margin of the lips. Amphibia and fish have 

 only small, disseminated buccal glands. In insects the salivary glands are widely 

 distributed, partly unicellular (as, for example, two pairs in lice), partly com- 

 pound; several pairs of them are usually present. In some the secretion contains 

 formic acid, for which reason the stings of these animals cause burning and in- 

 flammation; in others the secretion is strongly alkaline, as that from the large 

 salivary glands of the bed-bug. In bees and ants the lower salivary glands secrete 

 a sort of cement-substance. The web-glands on the lower lip of caterpillars 

 secreting the silky material, principally those of the silk-worm, should not be 

 confounded with the salivary glands. Among vermes, leeches have unicellular 

 salivary glands. In snails the salivary glands are also widely disseminated, 

 and the saliva from dolium galea contains more than 3^ per cent, sulphuric acid, 

 which also is present in murex. cassis, and aplysia. Cephalopods have a 

 double set of salivary glands. In the octopus the saliva digests fibrin, but not 

 starch, and it is poisonous. 



Crop-like formations are wanting in all mammals; the stomach appears to 

 be single, as in human beings, or divided into halves, as in many rodents, into 

 a cardiac portion and a pyloric portion. 



The stomach of ruminants consists of four portions: the first and largest 

 is the paunch (rumen), the next the honeycomb-bag (reticulum). In these two 

 portions, principally in the paunch, the ingesta undergo maceration and fermenta- 

 tion. They are now returned to the mouth by the action of the voluntary mus- 

 cular fibers passing to the stomach, again thoroughly masticated, and, by the 

 closure of a special semicircular groove (esophageal groove), the bolus is carried 



