THE MAMMALIA—MAN AND BEASTS. 



69 



only from three to five times their length among the Carnivora. In omnivorous 

 animals, such as Man, its length is intermediate to these, being six or seven times that 

 of the body. 



Several experiments have been made by Sir Astley Cooper to ascertain the diges- 

 tive power of the Dog. He found that this animal digested pork more easily than 

 mutton, the latter more rapidly than veal, and beef with greater diflSculty than any 

 of the others. He found that fish and cheese were easily digested by the Dog, and 

 boiled veal more readily than roast. The fat of meat seemed more digestible than 

 cheese, codfish dissolved more readily than beef, and beef than potato. The order 

 of digestion for the different parts of the same kind of animal food was fat, muscle, 

 skin, cartilage, tendon, and bone, the last being the least digestible. 



Young Dogs, when they have acquired strength, and are in good health, can 

 dio-est bones ; and what is remarkable, Spallanzani has observed that the gastric juice 

 of their stomachs made an impression even upon the enamel of teeth. Boerhaave 

 asserts the contrary, but the observations of Spallanzani have been confirmed. This 

 power of dissolving bone is not peculiar to the Mammalia, but is also possessed by 

 some animals of the other classes. Thus the Falcons, Eagles, and Crows, usually 

 refuse bones; but when introduced into their stomachs, with proper precautions, 

 these refractory substances are digested. Serpents and Adders also digest them, as 

 has been remarked by Spallanzani. Only the smaller bones, however, possessing the 

 least solidity, are dissolved entirely and rapidly ; the harder bones require to be 

 minutely divided in order to be softened and dissolved, otherwise they merely undergo 

 a small loss of substance. It must be observed, that before digesting they pass into 

 a cartilaginous state, and resemble indurated gelatine, as if they had been submitted 

 to the action of nitric acid. 



The Ruminantia, like the granivorous Birds, can digest herbs and grains, only 

 when these substances have been previously divided, mashed, or ground. When 

 entire herbs and solid grains are introduced into their stomachs, whether uncovered, 

 or inclosed in linen, or perforated tubes, these substances undergo no digestion; they 

 are merely moistened or softened, and this is the extent of their modification. The 

 same result is obtained even when they are moistened with saliva. On the contrary, 

 if bags or tubes of mashed herbs or grains be introduced into the stomachs of the 

 Ruminants, the digestion is then performed in a few hoxirs. These experiments have 

 been made upon Oxen, Sheep, and many other Ruminantia, and they present similar 

 results for all this order of jMammalia. They have also been made upon the Horse, 

 and with the same result, although he does not ruminate. 



Many physiologists have attempted, with various success, to effect artificial diges- 

 tions out of the body, by extracting the gastric juices from the stomachs of different 

 animals, and afterwards mixing them with the food. Spallanzani, by these experi- 

 ments with the gastric juices of different animals, obtained several important results. 

 WTien cold, the gastric juice produced scarcely any effect ; it merely opposed putrefac- 

 tion, but did not exercise its dissolving and digestive power until it was raised to its 

 proper temperature. It did not act upon grains and herbs until they were ground, 

 mashed, and impregnated with saliva. The gastric juice of Jlan softened and seemed 

 to digest beef m about thirty-sis hours, when raised to a temperature equal to that 

 of the stomach. He also observed, that the gastric juice of one species often acted 

 upon a great variety of substances, and yet it did not always act upon substances 

 which could be dissolved by the gastric juice of another species. 



From these instances of artificial digestion, we may readily expect that the stomach, 

 being lubricated by the gastric juice, will continue to digest after the death of the 

 animal, and even it has been said to digest itself. Hunter first noticed the fact, that 

 the gastric juice will act upon the sides of the stomach after death, and to this cause 

 he attributed the erosions and perforations which are sometimes found in the stomachs 

 of human subjects. Spallanzani made several experiments upon Dogs and Cats with 

 a similar result. He caused the animals intended for trial to fast for a long time, 

 and then to be fed immediately before being killed. Their bodies were placed in 

 stoves which preserved their natural temperature, and in the course of a few hours 

 he foimd that the food in their stomachs was sensibly digested. 



The chyme, after being slowly formed on the surface of that alimentary mass which 

 the stomach contains, accumulates as it forms near the pylorus or intestinal opening 

 of the stomach. It is raised from this situation into the narrow pyloric entrance, by 

 the increased action of those gentle and almost insensible contractions which it had 

 already experienced. The more violent contractions which are necessary to expel the 

 chyme from the cavity of the stomach, usually originate in the duodenum or small 

 intestine, from which thoy are transmitted to the pylorus, and thence gradually to the 

 entire stomach. But this ascending movement of the chyme, during which some 

 persons experience much uneasiness, is immediately converted into another movement 

 :n the opposite direction; and it is by means of this reaction that the chyme finally 

 traverses the pylorus, which is opened for that purpose, and by the same cause. 

 The movement is repeated, and continues as often as the quantity of chyme newly 

 formed in the stomach requires to be removed. In this way, the duodenum is gra- 

 dually filled in small quantities at each operation by the food chymified in the stomach. 

 Afterwards the course of the chyme is very slow in the intestineSj and the same ob- 

 servation applies to the intestinal movements, generally, except when excited by 

 disease or mental emotion. 



The fulness of the duodenum favors the secretion of the bile and pancreatic juices 

 as well as of mucus and the intestinal juices. All these new fluids being mixed with 

 the chyme immediately change its nature. It almost wholly loses its aciditv ; its 

 colour changes from gray to yellow, and it becomes bitter to the taste. This bitter- 

 ness in some animals extends even to the stomachic product itself, especially in 

 Birds, because the bile often penetrates through the pyloric entrance into the stomach. 

 If the aliments contain fat or oily matters, these substances pass into the intestine 

 without having undergone any alteration in the stomach, but the bile unitinn- with 

 them, forms a kind of soap, which is easily dissolved. AVith the exception of herbs, 

 andlhesefatand oily substances, the duodenum permits all bodies, which the stomach 

 has not previously digested, to pass without alteration. The same aliments produce a 

 siradar chyrae in animals of the same species when in health, and the changes which 

 tlie chyme afterwards undergoes are equally the same. But the chyme produced 

 18 



from animal food is thicker and more viscous than that produced from vegetables ; 

 it is reddish, and does not curdle milk. Vegetable chyme, on the contrary, is al- 

 most fluid ; it has a yellowish tinge, and curdles milk. Further, the chyme fur- 

 nished by vegetables is less rich in nutritious matter, and it wants that albuminous 

 substance, which is found in chyme resulting from animal food. Different kinds of 

 gas are disengaged during the process, but their nature varies according to the species 

 of animal, its age and state of health, the kind of food which is used, and especially 

 according to the part of the intestinal canal from which they proceed. 



In a short time after the chyme has descended into the duodenum, and undergone 

 the action of the bile, it divides into two parts. The one is a fluid termed c%/e, 

 which is the part destined to nourish the animal ; the other is more solid and coarse, 

 and less homogeneous, and, being the useless residue of the aliment, is finally re- 

 jected. 



This separation of the chyle, and even its formation, appear to be more especially 

 due to the influence of the bile ; at least it is certain that digestion is always im- 

 perfect, and that the chyle is either deficient or in small quantity, when the bile can- 

 not mix with the chyme prepared by the stomach. 



Among the greater part of the Mammalia, the time which the chyle takes to form, 

 after the chyme has passed from the stomach into the duodenum, varies from two to 

 four hours. This function is much slower among the Fishes, and still more so with 

 the Reptiles. Well formed chyle has sometimes been found in the white vessels which 

 adjoin the stomach, and it has been said in consequence, that the stomach can form 

 chyle. But this has only been seen in certain animals, whose bile frequently mingles 

 with the gastric juice. The Dog appears to be an instance ; but it does not seem to 

 bo of frequent occurrence among the Blammalia generally. 



" Les excremens," observes M. Isidore Bourdon, '* separes du chyle qui les sur- 

 nage et dont I'absorption s'opere dans le haut de I'intestin, perdent peu-a-peu, ;i mc- 

 sure qu'ils descendent vers les gros intestins, la fluidite qu'ils avaient dans le milieu do 

 1 intestin grele, Le mucus des gros intestins en favorise la marche vers I'anus, raais 

 les loges que presentent ccs conduits de distance en distance, en prolongent le sejour 

 et en accrolssent la consistance. C'est par Taction des fibres musculeuses des intes- 

 tins que les excremens son peu-a-peu pousses vers I'anus, et c'est par les muscles ab- 

 dominaux qu'ils sent finalement rcjetcs hors du corps. Cette expulsion resulte d'un 

 mecanisme assez complique ou la glotte, au moins chez les Mamiferes, joue un roie 

 important. Le rejet des mati^res fccales est beaucoup plus facile chez les animaux 

 ovipares et dans I'Ornithorrhynque ; et ce'te difference resulte de ce que cesanimaus, 

 ayant un cloaquc, leurs urines s'amassent dans ce lieu aussi bien quo les excremens ; 

 qu'elles delayent. Les excremens different pour chaque espece d'animal ; mais la 

 plus grande difference s'observe surtout entre les carnivores et les herbivores. Lo 

 meme animal, s'll est omnivore, a des excremens tres differens, suivant qu'il use 

 d'alimens vegetaux ou d'alimens tires de I'autre regne. Les excremens provenant 

 dune nourriture animale out la propriete de faire cailler le lait, et il n'existe rien do 

 semblable pour les feces des alimens vegetaux. C'est absolument le contraire de ce 

 que nous avons dit pour le chyme des carnivores et des herbivores." 



Bordeu has made several interesting remarks on this subject; the cm'iosity of an 

 ingenious mind having overcome the natural disgust towards a study so repulsive. 

 The researches of Prout are more precise than those of Bordeu. 



Comphcated fluids are digested as well as solids, but a largo part of them are -ib- 

 sorbed directly by the stomach. AVatcr, alcohol, and other simple fluids, contribute 

 towards nutrition, chiefly by imparting their fluidity to some of the animal bodies. 



Those movements of the intestines by means of which they are traversed by the 

 stomachic product, are termed peristaltic. There are, however, other movements, 

 which are directed from below, upwards, in the contrai-y direction, and are hence 

 termed anti-peristaltic. These movements produce various phenomena, such as Re- 

 gurgitation, Rumination, and Vomiting. Among the Mammalia no animal vomits 

 more readily than the Cat; there are, however, few Vertebrated animals which do 

 not possess the power of vomiting. The Horse cannot vomit in ordinary cases, be- 

 cause the situation of the cardia opposes an obstacle to the return of the food towards 

 the aisophagus. 



It is ascertained that the chyle separates itself from the alimentary mass, after it 

 has remained for some time in the distended cavity of the duodenum, and that in 

 a short time the bile and pancreatic juices act upon it, although we are ignorant of 

 the precise nature of the operation. With respect to the characters and quality of 

 the chyle, this substance is very plentiful in the JIammalia alone. The chyle is 

 always of an opaque white, which has caused it to be compared to milk. When 

 taken from the body, and left to itself, it separates into two portions ; the one is 

 serous and saline, while the other is fibrous, in this respect nearly resembling the 

 blood when similarly treated. If placed in an inert vessel, the chyle usually acquires 

 a reddish tinge, which appears to be owing to the action of the oxygen in the air ; 

 and a thick kind of cream forms on the surface. This resemblance of the chyme to 

 milk has led some physiologists to consider the one as the product of the other. One 

 thin"- is certain, that nothing tends more towards the abundant secretion of milk, in 

 the female, than that plentiful production of chyle resulting from the abundant sup- 

 ply of nutritious food. In other respects, the nature of the food with which the ani- 

 mal is nourished greatly influences the chjde resulting from digestion. Different 

 substances do not produce the game kind of chyle; fat matters produce a chyle which 

 is white, and more opaque than that yielded by substances which are not fat. The 

 chyme never acquires the hue of any colouring matter which may have been intro- 

 duced into the intestine; and it is even with difficulty that it can be made to acquire 

 an odour. 



The chyle, when once separated from the chyme, of which it may be regarded as 

 the extract, floats on the surface of this' matter, and accumulates by small rivulets in 

 the mucous valvules, with which the interior of the smaller intestines are supplied. It 

 remains in this place for a few instants, when it is absorbed by the small vessels, 

 which serve to bring it hito the mass of the blood. It is impossible to say in precisely 

 what manner, or by what force or mechanism, this absorption of the chyle is effected, 

 and to indulge in conjecture would but lead to certain error. On carefully examining, 

 with the naked eye, the interior cf the intestine, at the moment when the chyle is 



