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Ruminants are known for their regurgitation of food; the food goes through the 1st and 2nd stomach chambers, then returns to the oral cavity in cuds. The cuds then come back into the stomach, but this time they go through the 3rd and 4th chambers, instead of going to the 1st and 2nd again.

How is it ensured that the regurgitated food doesn't go through the 1st and 2nd chamber by a mistake?

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Your question supposes a very linear version of the flow of stomach contents of:

1->2->mouth->3->4

In reality, it is not so linear. Firstly some background: The rumen and reticulum form the first two parts of the four, and are in fact a single chamber/organ (collectively known as the reticulorumen). They have different wall structures, and perform initial digestion of different particles (mostly heavier particles in the rumen such as grain and more digested hay). This reticulorumen chamber is where the initial microbial fermentation of the feed occurs. Matter passes from rumen to reticulum and back regularly by means of two distinct muscle contractions.

To aid in the breakdown of the feed, the cud is regularly regurgitated, chewed and then re-ingested. The re-ingested cud returns to the reticulorumen. From there, in order to advance down into the later stages of the digestive tract, it must pass through the reticulorumen orifice into the third compartment - the omasum. The orifice allows particles up to 4mm in size in cattle.

The omasum chamber removes water and some nutrients. The contractions of the omasum remove water and move solids through the system, and the contractions are speculated to filter out any larger particles and return them to the reticulorumen. Matter that has been processed by the omasum is passed along to the fourth chamber, the abomasum.

So to answer your question in brief, after mastication the cud returns to chambers one and two, potentially going round this loop several times until it is sufficiently broken down by chewing, saliva and bacterial action, and then it can proceed from there to the third chamber.

The "digesta circulation and reticulo rumen motility in the ruminent" paper (C. Poncet, C.H. Malbert, R. Baumont) has this to say:

The outflow of digesta, allowed by the opening of the ROO [reticulorumen orifice] during the second phase of the reticular contraction, is highly selective. The effluent does not contain particles greater than 2mm in sheep and 4mm in cattle. This is due to the buoyancy of the large particles in the reticulum, to the self filtration of the digesta during the passage through the ROO and perhaps to backflows from the omasum to the reticulum.

You may find it useful to get a copy of this collection of papers (quote from p.366): http://books.google.co.uk/books?id=Bcsa8Z4u-E4C&printsec=frontcover#v=onepage&q&f=false

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Thanks. Just to make sure I understand, do you mean that the large pieces of food simply cannot get through the passage to the 3rd chamber, and only the ingested, chewed-into-small-pieces food manages to get through? –  Greek Fellows Apr 20 at 13:41
    
In the main yes, although all digestion will be a little inexact - the odd larger particle could in theory get through I would guess. The size of the reticulorumen orifice would place an upper limit on the size of particle that could pass, and it is believed that the omasum will pass back some larger particles - the omasum contracts to expel water and move matter, and this may work in both directions. –  Peter Bailey Apr 20 at 18:33
    
If you found my answer complete, please mark it as accepted. If you do not find it sufficient, I'm happy to clarify any points. Thanks. –  Peter Bailey Jul 20 at 22:14
    
I am very sorry... –  Greek Fellows Jul 22 at 3:47
    
No problem, thanks! –  Peter Bailey Jul 22 at 20:33

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