The surface of the small intestine is increased for the purposes of absorption by the folds of mucous membrane called the valvula conniventes, and also by the smaller projections called villi. Though these are absent from the large intestine, absorption particularly of water goes on there, and the contents thus become of more solid consistency as they approach the rectum. The length and capacity of the intestines vary much in different animals, and are related to the bulk of food these animals are in the habit of ingesting. The length of the intestines is thus much greater in herbivora than in carnivora. The horse is an exception to this rule, but the immense capacity of its intestines makes up for their shortness. Omnivorous animals, like man and the pig, occupy an intermediate position between carnivora and herbivora. The following table gives some particulars bearing out the above general statements :

CHAPTER XXXII

THE SECRETION OF THE PANCREAS

THE pancreas is a gland very similar in structure to the parotid gland. Its duct enters the duodenum close to the orifice of the bile-duct.

Such knowledge as we possess of the chemical composition of the pancreas as a whole has been given on p. 558. We are now more particularly interested in its secretion.

The older physiologists were quite ignorant of the vast importance of the pancreatic juice. Claude Bernard (1846) considered that it was instrumental in the emulsifying of fats. Bidder and Schmidt1 were the first to make analyses of the juice, and our knowledge of its action and its ferments is due to the investigations of Heidenhain,2 Bernstein,3 Langley, Lea, and especially Kühne.6

4

5

The dog has been the animal from which pancreatic juice has been generally obtained, as the principal duct of the pancreas in this animal enters the intestine quite two centimetres from the orifice of the bileduct. A cannula is inserted into this duct, brought through the abdominal wound and carefully stitched to it; in a few days the wound heals (Cl. Bernard). The animal suffers from not being able to carry on intestinal digestion properly, and in consequence the pancreatic juice in a day or two becomes very watery compared to that which is secreted at first.

The secretion of pancreatic juice begins in the dog immediately after the introduction of food into the stomach, and attains a maximum three hours later (Bernstein). A large amount of food increases both the quantity and the quality of the juice secreted; the juice, however, secreted at the commencement of digestion is always richer in solid constituents than that secreted later. The secretion of pancreatic juice is stated to be continuous in herbivorous animals (Heidenhain 7).

There is at present nothing known concerning secreting nerves of 1 Die Verdauungssäfte und der Stoffwechsel, Mitau and Leipzig, 1852; Ann. Chem. Pharm. xcii. 33.

2 Pflüger's Archiv, x. 557.

3 Sitzungsb. d. Akad. d. Wiss. (Leipzig, 1869), p. 96. 4 Journ. Physiol. iii.

5 Kühne and Lea, Verhandl. d. Heidelberg, naturhist. med. Vereins, N.F. 1, Heft v.

P. 445.

6 Arch. f. path. Anat. xxxix. 130; Heidelberg. Verhandlungen, N.F. 1. Heft iv. and v. 7 Pflüger's Archiv, xiv. 457.

the pancreas. During digestion the pancreas, however, like the other abdominal organs, is gorged with blood from dilatation of its vessels. Induction shocks applied to the organ itself or injections of blood or chyle stimulate the secretion (Kühne and Lea).

Pancreatic juice is secreted under considerable pressure; in the rabbit the pressure in the duct amounts to 16 to 17 mm. of mercury (Heidenhain).

The quantity secreted by the dog is about 25 grammes per kilogramme of body-weight in the twenty-four hours (Bidder and Schmidt). Colin obtained from the horse 175, from the cow 200 to 270, from the pig 12 to 15 grammes per hour. It has been calculated that a man secretes about 150 grammes of pancreatic juice per diem.

Microscopic examination of the gland-cells in different stages of activity reveals a series of changes comparable to those already described in the case of the salivary and gastric cells. Granules indicating the presence of a zymogen,' which is called trypsinogen

FIG. 85.- Part of an Alveolus of the Rabbit's Pancreas: A, before discharge; B, after.
(From Foster, after Kühne and Lea.)

(i.e. the precursor of trypsin, the most important ferment of the pancreatic juice), crowd the cells before secretion; these are discharged during secretion, so that in an animal whose pancreas has been powerfully stimulated to secrete, as by the administration of pilocarpine, the granules are only seen at the free border of the cells (Kühne and Lea).

For the investigation of the action of pancreatic juice, an artificial juice is now usually employed. A pancreas is allowed to stand at the ordinary temperature of the air for twenty-four hours; or it may be treated with dilute acetic acid immediately; either method converts the zymogen into the ferment. It is then minced and placed for some days or weeks under glycerin.2 The glycerin dissolves out the

1 Refer to p. 451. These granules are not so readily destroyed by chromic acid as those in the salivary glands and stomach.

ferments; these may be precipitated from the extract by alcohol, then collected, dried at a low temperature, and preserved for future use. An artificial pancreatic juice may be then made by dissolving this in 1 per cent. sodium carbonate solution; or a little of the glycerin extract may itself be added to the same alkaline solution; this, however, acts more slowly because of the presence of glycerin.

COMPOSITION OF PANCREATIC JUICE

The normal secretion of the pancreas in the dog is a clear, colourless, viscid, almost syrupy fluid. It has a saltish taste and strong, alkaline reaction. The alkalinity is due to phosphates and carbonates, especially of soda. The pancreatic juice of herbivora is more watery than in the dog; in one case of human pancreatic juice obtained by Herter the fluid was not viscid but limpid.

The following analyses (given in parts per 1000) have been made:

The organic substances present in the pancreatic ferment are :a. Ferments: These are the most important, both quantitatively and functionally, of all the constituents; they are four in number: i. Trypsin-a proteolytic ferment.

ii. Amylopsin or pancreatic diastase an amylolytic ferment. iii. Steapsin a fat-splitting ferment.

iv. A milk-curdling ferment.

1 Diss. Dorpat, 1854. Quoted from Hoppe-Seyler, Physiol. Chem. p. 259.

2 Ibid.

P.

259.

3 Quoted from McKendrick's Physiology, ii. 125.

b. A small amount of proteid which is coagulable by heat. c. A mucin or mucin-like substance.1

d. Traces of leucine, tyrosine, xanthine, and of soaps have been described.

The ferments of the pancreatic juice.-i. Trypsin. --Bernard2 and later Corvisart 3 observed that the pancreatic secretion dissolved coagulated white of egg. Kühne studied this action carefully and gave the name trypsin to the ferment that produces the action. Kühne prepared the ferment by means of making an aqueous extract of the pancreas at 0° C., and precipitating the proteids and ferments therefrom with alcohol. The precipitate was collected, dissolved in water, and acetic acid added till 1 per cent. of the acid was present in the solution; the precipitate so produced was again extracted with water and filtered; the filtrate was again treated in the same way, first with alcohol, then with acetic acid; the filtrate was made alkaline with soda, digested at 40° C., and filtered. The filtrate was evaporated down, and thus tyrosine crystallised out; the rest of the tyrosine, leucine, and peptone was dialysed off. Though this method gives a purer ferment than those previously adopted, yet, as Hoppe-Seyler states, the preparation cannot be regarded as absolutely pure. The substance obtained is soluble in water and in glycerin, but not in alcohol; when a solution in water is acidified faintly and heated, a heat-coagulum is formed.

Kühne has more recently introduced the following method of preparing trypsin. The fresh or dried gland is first digested with 0.1 per cent. solution of salicylic acid for four hours, then with alkaline solution of thymol for twelve hours; the acid and alkaline extracts are mixed and the amount of thymol brought up to 0.5 per cent.; the amount of soda is brought up to the same percentage and the mixture is digested for six days, then cooled, and the tyrosine crystals which have formed are filtered off. It is then neutralised with acetic acid and saturated with ammonium sulphate; this precipitates the trypsin ; the precipitate is collected, washed with saturated solution of ammonium sulphate, and dissolved in 0-2 per cent. soda solution; this gives a powerful digestive fluid. If one desires to get rid of ammonium sulphate, this is done by dialysis.

A conclusion which appears to be justified from these methods of

1 In two specimens of dogs' pancreatic juice I have examined, acetic acid gave a stringy precipitate. The viscidity of the juice is evidently due to this substance, though whether it is true mucin or a nucleo-albumin I did not investigate.

2 Leçons, Paris, 1855, p. 334.

3 Sur une fonction peu connue du pancréas, Paris, 1858.

4 Centralbl. med. Wiss. 1886, No. 3

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