288 COMPARATIVE PHYSIOLOGY OF DIGESTION. 



masses in constipation are usually hard and dry, owing to the water being absorbed ; hence 

 they form large masses or scybala within the large intestine, and these again give rise to new 

 resistance. Amongst the reagents which prevent evacuation of the bowels, some paralyse the 

 motor apparatus temporarily, e.g., opium, morphia ; some diminish the secretion of the intestinal 

 mucous membrane, and cause constriction of the blood-vessels, as tannic acid, vegetables con- 

 taining tannin, alum, chalk, lead acetate, silver nitrate, bismuth nitrate. 



J. Increased evacuation of the intestinal contents is usually accompanied by a watery condition 

 of the faeces, constituting diarrhoea. The causes are : 



1. A too rapid movement of the contents through the intestine, chiefly through the large 

 intestine, so that there is not time for the normal amount of absorption to take place. The 

 increased peristalsis depends upon stimulation of the motor-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 occur (fig. 144, c). In ulceration of the intestine, leucocytes (pus) are 

 present {Nothnagel). 



3. Diarrhoea 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 intestina' mucous membrane. [Irritation over the abdomen, as from 

 the subcutaneous injection of small quantities of saline solutions, causes diarrhoea.] 



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

 {Masloff). 



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

 tatively and qualitatively, with simultaneous alteration of the functional activity of the muscul- 

 ature 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 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, but the parotid is absent. 

 Amougst 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 developed in molluscs, and the saliva 

 of Dolium galea contains more than 3 per cent, 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 intestine 

 is short in flesh-eating animals and long in herbivora. The stomach of ruminants is compound, 

 and consists of four cavities. The first and largest is the paunch or rumen, then the reticulum. 

 In these two cavities, especially the former, the ingesta are softened and undergo fermentation. 

 They are then returned to the mouth by the action of the voluntary muscular fibres, which 

 reach to the stomach. This is the process of rumination. The ingesta are chewed again in the 

 mouth, and are again swallowed, but this time they enter the third cavity or psalterium 

 (which is absent in the camel) and thence into the fourth stomach or abomasum, in which 

 the fermentative digestion takes place. 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 secre- 

 tion "pigeon's milk," which is used to feed the young (J. Hunter). The stomach consists of 

 a glandular proventriculus and a strong muscular stomach which is covered with horny epi- 

 thelium and triturates the food. There are usually two fluid diverticula on the small intestine 

 near where it joins the large gut. In fishes the intestinal canal is generally simple ; the stomach 

 is merely a dilatation of the tube ; and at the pylorus there may be one, but usually many, 

 blind glandular appendages (the appendices pyloricae). They are generally longitudinal folds in 



