COMPARATIVE PHYSIOLOGY OF DIGESTION. 383 



nervous apparatus of the intestine, usually of a reflex nature. Rapid transit of the 

 contents through the intestine causes the evacuation of certain substances, which 

 cannot be digested in so short a time. 



2. The stools become thinner from the presence of much water, mucus, and the 

 admixture with fat, and by eating fruit and vegetables. In rare cases, when the 

 evacuations contain much mucin, Charcot's crystals (Fig. 115, c) occur. In ulcera- 

 tion of the intestine, leucocytes (pus) are present (Nothnagel). 



3. Diarrhoaa may occur as a consequence of disturbance of the diffusion-processes 

 through the intestinal walls, as in affections of the epithelium, when it becomes 

 swollen in inflammatory or catarrhal conditions of the intestinal mucous membrane. 

 [Irritation over the abdomen, as from the subcutaneous injection of small quan- 

 tities of saline solutions, causes diarrhoea (M. Hay).] 



4. It may also be due to increased secretion into the intestine, as in capillary 

 diffusion, when magnesium sulphate in the intestine attracts water from the 

 blood. 



The same occurs in cholera, when the stools are copious and of a rice-water 

 character, and are loaded with epithelial cells from the villi. The transudation 

 into the intestine is so great that the blood in the arteries becomes very thick, 

 and may even on this account cease to circulate. 



Transudation into the intestine also takes place as a consequence of paralysis of 

 the vaso-motor nerves of the intestine. This is perhaps the case in diarrhoea 

 following upon a cold. Certain substances seem directly to excite the secretory 

 organs of the intestines or their nerves, such as the drastic purgatives (p. 364). 

 Pilocarpin injected into the blood causes great secretion (Masloff). 



During febrile conditions, the secretion of the intestinal glands seems to be 

 altered quantitatively and qualitatively, with simultaneous alteration of the 

 functional activity of the musculature and the organs of absorption, while the 

 excitability of the mucous membrane is increased (Uffelmann). It is important to 

 note that in many acute febrile diseases, the amount of common salt in the urine 

 diminishes, and increases again as the fever subsides. 



187. Comparative. 



Salivary Glands. Amongst Mammals the herbivora have larger salivary 

 glands than the carnivora ; while midway between both are the omnivora. The 

 whale has no salivary glands. The pinnipedia have a small parotid, which is 

 absent in the echidna. The dog and many carnivora have a special gland lying 

 in the orbit, the orbital or'zygomatic gland. In Birds the salivary glands open at 

 the angle of the mouth, in them the parotid is absent. Amongst Reptiles the 

 parotid of some species is so changed as to form poison glands ; the tortoise has 

 sublingual glands ; reptiles have labial glands. The Amphibia and Fishes have 

 merely small glands scattered over the mouth. The salivary glands are large in 

 Insects ; some of them secrete formic acid. The salivary glands are well de- 

 veloped in molluscs, and the saliva of dolium galea contains more than 3 p. c. of 

 free sulphuric acid (?) The cephalopods have double glands. 



A Crop is not present in any mammal ; the stomach is either simple, as in man, 

 or, as in many rodents, it is divided into two halves, into a cardiac and a pyloric 

 portion. The stomach of ruminants is compound, and consists of four cavities. 

 The intestine is short in flesh-eating animals and long in herbivora. The caecum is 

 a very large and important digestive organ in herbivora, and in most rodents; 

 it is small in man, and absent in carnivora. The oesophagus in grain-eating 

 Birds not unfrequently has a blind diverticulum or crop for softening the food. 

 In the crop of pigeons during the breeding season, there is formed a peculiar 

 secretion "pigeon's milk," which is used to feed the young (J. Hunter). The 



