Bovine biology series
Part - 28 Jaw, teeth and palate
One of my very early remembrances of cleaning the metal feed pans in the milking parlor was the discovery of a cow's tooth. I remember washing it with soapy water and keeping it in my pocket for some time. After several more of these discoveries I soon lost interest in collecting these dull white objects, although these discoveries served their role in placing in my mind the wonder of biology.
I recall my discovery that cows did not have upper teeth, instead, a palate of hard tissue comprised the upper portion of the mouth. The lower portion, comprising the jawbone and its row of aligned teeth, were obviously the other side of the grinding motion that I knew was required in cud-chewing.
I wondered for a long time why cows would move their jaws in circular motions, and upon the discovery of the above teeth and palate, the duality into the single function of reducing feed and primarily forage into smaller particles became obvious.
Somewhere along the way, the original cows teeth, the prized possessions of a youngster that seemed so valuable at the time, were lost. Kept, however, was the fascination of them.
The jawbone of the cow, of which there are two that come together in the front of the mouth, are L shaped bones that can be as short as 9 inches for a Jersey to over 14 inches long in a mature Holstein. The jawbone is anchored in the skull. Strong cartilage, that tough substance that connects soft tissue with bone, holds the jaw in relative place. Like most joints of bone and tissue, the position of them is relative, so that flexibility can move them depending upon need. The relative positioning allows the joint to adsorb shock, otherwise, bone fractures might occur readily.
The jaw bone is true bone, a specialized connective tissue that is, like all bone, tissue that has been mineralized with calcium and phosphorus, and thus serve the role in the body as a "sink" for these two important minerals. Once in the sink, so to speak, they are rather permanent, for in the metabolism of bodily tissue, the bone outlasts them all. And with all bone there is an organic component. Here, there is collagen, a protein that comprises a fibrous tissue that aids blood flow into bone tissue, so that bones can be refurbished with nutrients.
Bones, like the jawbone, are about 25 percent water, 45 percent ash, and 30 percent organic matter. Thus when bone is dried, the remaining dry matter is two thirds mineral.
The jawbone is hollow, like many bones, in that within the tunnel of the jawbone are long fibers of nerve cells. Some of these nerve cells end at the root area of the teeth, where the teeth are anchored on the jawbone.
The teeth of a cow are large molars and incisors, and as such, there are not anchor teeth, or canine teeth. We may interpret this fact by knowing that cows are not meat-eaters, thus, they do not need the longer, sharper canine to dig into meaty flesh so that it may be torn from the tissue. As herbivores-plant eaters, the tissue in plants and grains is softer, so a lineup of molars and incisors is all that is required.
The teeth of ruminants are complex, or we may use the word hypsodont, which means "having prism-shaped teeth with high crowns", describing the teeth of plant eating animals. They originate in epithelial and connective tissue, from inside the dental lamina just above the jawbone.
The first to form are the deciduous teeth, or baby teeth. These are soft teeth without the enamel that hard teeth possess. Later the permanent teeth begin to grow, and as they do, shove their roots with a downward pressure against the jawbone. Because the jawbone is rigid, the downward force causes the tooth to erupt, or force itself upward toward the mouth cavity. This upward movement, or eruption, causes the crown of the permanent tooth to rise further into the mouth; thus, the white teeth are exposed and can now participate in their primary function....chewing.
As teeth mature, the roots become further associated with the jawbone, providing stability and a way for nutrients and nerves to keep the tooth healthy. Teeth are supplied with blood and nerve endings; they are alive and constantly in a state of flux, just like all tissue. The fact that they are primarily mineralized tissues and rock hard is a function of their purpose.....chewing, and does not mean they are stagnant, or dead tissue.
The teeth I found in the feed pan were lost, probably, from an older cow in which the toll of daily grinding for years had loosened the root grip on the jawbone. Perhaps the blood vessels and nerve endings were, over the course of time, reduced in number, and so one day the looseness became sufficient that the tooth simply fell out.
The dental pad, or palate, is found on the upper part of the mouth cavity. This tissue is similar to other specialized tissue of the body in that it is keratinized epithelial tissue. The term "keratinized" means tissue that is mostly protein chemically. The protein molecules are intertwined so completely that the resulting tissue is hard, tough, and durable. Further examples of keratinized tissue are hair, hooves, nails, and horns.
The dental pad is solidly anchored in the bony roof of the mouth. The top of the mouth, this dental pad and bone, provides the offsetting complement to the jawbone, so that the upper and lower portion of the mouth have continuity. As such, the dental pad and bone are anchored in the skull permanentlier than the jawbone.
So when a cow chews, the upper portion of the mouth is the stationary portion; the lower jawbone is moveable, less permanent, and more flexible, so that feed can be physically reduced in size. The back and forth action of the jawbone and lower teeth against the stationary palate provides the ideal chewing mechanism for plant eaters. And in the cud-chewing action, the cow simply grinds further what the rumen sends to it, all with the purpose of reducing physical size.
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