10.21.96 -----begin lecture-----

handout: COMPLEX DIGESTION SYSTEMS

herbivores eg equus, bovis, etc, do not have the enzyme cellulase. 
therefore they need another way to break down cellulose into glucose. so, 
they have microorganisms in the reticulorumen which make cellulase, which 
in turn breaks down the cellulose. herbivores do digest part of their 
food in the abomasum as well, but must digest the cellulose via the 
microbes.

so, nature has two solutions: 

one, put a fermentation vat in the beginning, eg goats, sheep, cows 
(rumen) or 
two, put a fermentation vat in the back, eg horses, where the cecum and 
ascending colon function as a fermentation vat at the back end of the 
animal.

a ruminant stomach is possessed by animals that ruminate - eg, chew cud, 
regurgitate out of reticulorumen and chew food again then reswallow it. 

true ruminants (RUMENANTIA) consist of deer, giraffes, bovids (which 
include sheep and goats).

there are also PSEUDORUMINANTS which chew cud: camels, llamas, alpacas - 
these animals have 3 chambered stomach, whereas true rumenants have 4 
chambers.

there are some other animals with capacious, sacculated stomachs which 
break down cellulose - leaf eating monkey, hippopotomus - but they are 
not truly compartmentalized, they do not chew cud, not as efficient as 
rumenantia.

Church, et al. The Ruminant Animal: on reserve. read if you wanna be a 
cow doctor or if you need more background on ruminants.

Now, ruminants choose a variety of plant material to eat. Deer, moose = 
concentrate selectors. these animals handle more succulent material. 
cattle, sheep eat very fibrous material = grass/roughage eater. the 
animals which eat grasses and roughage have more complex GI systems, more 
nooks and crannies, and they can feed less frequently than say deer, 
because the concentrate selectors have less complex gi and faster transit 
time through the rumen. however, the large intestine of the concentrate 
selector (CS) is much larger than that of the grass/roughage eater (GR). 
so, the CS can have further breakdown of cellulose inthe large distal 
fermentation chamber (DFC).

note: cow nose is flat, hard surface, doesn't twitch around like 
horse/sheep/goat. very flat and rigid. what a cow does is just to put its 
head down, and use tongue to rip out grass and just swallow it.

ruminant doesn't have four stomachs, it has a four chambered stomach. so 
there is a forestomach consisting of the reticulum, the rumen, and the 
omasum, and behind that is the abomasum, which is glandular like our 
simple stomach. the forestomach --->campbell's pepperpot soup, tripe. [he 
now described trippe a la lyonnaise, which sounded gross to me] [also 
haggis]

[slide- cow stomach - reticulum very cranial,reticulorumenal groove,  L 
longitudinal groove, etc etc.]
[slide- cow stomach - R side. ]
the point is that the grooves indicate some separation of the 
compartments.
[slide- cutaway cow]

so, the right dorsal coronary pillar is indicated on the surface by a 
coronary groove. there's a caudal pillar, and a cranial pillar, etc. 
these pillars are tough and muscular, and they serve as kind of a 
skeleton for this organ to contract against, and they contract as well. 
note also that the reticulum, or cranial part of reticulorumen, lies at 
the bottom of the reticular groove. when cow is full, rumen is about half 
full, and the dorsal half is a gas pocket. the reticulum lies just 
ventral to the opening of the esophagus and also lies just caudal to the 
diagphragm not far from the heart. so, sharp objects entering reticulum 
can stab the heart (or stab the diaphragm, which isn't nice either). can 
get traumatic reticulitis, hardware dz. if penetrates thorax, can see 
traumatic pericarditis - pus in the pericardium - very bad.

p 3 of handout fig 20.2 shows the gas pocket, and the distribution of 
heavy/dense feed and light feed. so, corn and oats tend to drop down to 
the bottom of the reticulum, while lighter things like hay and straw 
float around on the top. this is all churned around like in a washing 
machine.

so, you have the omasum and abomasum coming off the rumen, and food gets 
out of reticulorumen via a  very regular contraction cycle as outlined on 
p 3 of the handout.pressures were measured by using balloons in the 
stomach attached to pens. in diag 20.3 item 1b is the resting reticulum, 
so you see two short contractions in the reticulum, then a contraction in 
the cranial rumen, then in the dorsal sac, then in the caudodorsal blind 
sac, then in the ventral sac.

the ruminoreticular contraction cycle occurs about 1 time/minute. if you 
watch L paralumbar fossa of cow, you can see a bulging and relaxing. 
during the contracion, some of the material will be allowed out the 
rumenal/omasal opening.
recall that the omasum has many laminae of different lengths and is a 
great absorbing surface. it's not just a holding vat. there are enzymes 
in the epithelium, which is stratified squamous epi., and various 
nutrients are absorbed here or actively transported by enzymes into the 
cells.
so this churning action occuring once/min gets the food into a state 
where it can pass through the reticuloomasal orifice and eventually 
continue on.

also in fig 20.3 note the eructation contraction which occurs between 
primary contractions. this has to happen so that the animal can get rid 
of the gas building up in the rumen. if there is no eructation, the 
animal will die. the contraction pushes the gas forward, and there are 
special receptors at the cardia which tell the difference between gas and 
fluid, so the cows can belch ok. when a cow eructates, its mouth is 
closed, soft palate is raised, so the animal inhales the methane and CO2. 
why? maybe to hide from predators? if a cow is behind a rock and and 
animal is on the other side, perhaps it is better not to loudly belch 
into the air? <G>. See 1a of the same figure, this is a reticular tracing 
during rumination. rumination= regurgitation, remastication, 
re-ensalivation, reswallowing. when the animal is relaxed it will just 
chew its cud, then it will stop, then you can watch and you'll see what 
looks like a mouse running under a carpet in the jugular groove, and 
that's a new bolus of food going into the mouth, and then it will start 
chewing again.

the dorsal vagus innervates the ruminant stomach. if the rumen were 
paralyzed, the animal would die. to treat ulcers in humans, they cut the 
vagi, and people are ok. in ruminants, you can cut the ventral vagus, but 
if you lose the dorsal vagus, you're in big trouble. celiac artery 
supplies ruminant stomach.

a bit about development. what about lambs and calves and kids? well, the 
forestomach of a newborn is small compared to the abomasum, because milk 
will be digested in the abomasum. in the first few weeks after birth, the 
proportion of forestomach to abomasum switches. also, in neonates, there 
is a reflex which causes the lips of the reticular groove to flap such 
that incoming milk goes directly down into the abomasum instead of 
staying in the rumen. this is the reticular groove reflex. when the 
animal suckles, nerves in the caudal wall of the pharynx send impulses ( 
when stimulated by fluid hitting the wall of the pharynx up the cranial 
laryngeal nerve) to the medulla where the cranial laryngeal nerve 
synapses with motor nerve which sends impulse down vagus/dorsal vagus 
which synapses in turn with the muscles of the reticular groove to cause 
the reflexive closure. this happens whenever a suckling animal drinks 
liquid. as animal gets older, milk will stimulate reflex while water will 
not. can stimulate this reflex in sheep by using copper ions, even in 
adult, and in cows, sodium ions, will elicit reflex and bypass the rumen, 
say if you want to get meds into caudal GI tract right away...

what about the papillae of the rumen? what causes their development? 
well, they increase the surface area. it used to be thought that the 
roughage eaten induced development of the papillae, since they arne't 
present early in life, but that's been disproved. it seems that fatty 
acids, proprionic and butyric acid, etc - eg, breakdown products of 
roughage - induce the development of the papillae, which developed to 
better absorb these products. so a CS doesn't have as many papillae as a 
GR.

p 4 - diagrams of development of stomach. reticulorumen does NOT develop 
from esophagus. it develops from a dilatation of gut tube , from 
primitive stomach.

last two pages of handout, diagrams of intestines of dog, horse, pig, ox. 
note that the specializations are all analagous to the simple ascending 
colon of the dog, because they have the same blood supply from the 
cranial mesenteric artery.

one final thing to note, animals like rabbits and mice have another way 
of getting nutrients, and that is by coprophagia. rabbits will eat "night 
feces" or cecotropes directly from the anus. during the day, regular 
pellets are defecated. special, bigger, moister pellets are developed and 
stored in the cecum and defecated at night, and picked up by the lips and 
reswallowed. so you don't want to leave an e-collar on a rabbit too long, 
can get malnourished.  this is also a phenomenon of one year old 
children, we're told... <G>

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