Bovine biology series
Part - 26 Nose
The nose and nasal passage
One of my favorite photographs shows one of my mentors, Harmon Toone, an instructor, dairy judge, and friend to many, many dairy science students across the country. He is standing next to a famous cow, a Holstein of some genetic and production greatness. The cow has her nose in his hand. And upon his face is the comfort of her touch.
Certainly a bond exists between Harmon and this cow. The link, of course, is the palm of his hand and the sensitive, moist, and soft skinned cow's nose. I rather believe that if a cow or any other animal perceives the comfort of such linkage, then they will place their head in our hands. No doubt, this famous cow is comforted by his touch as well.
So what is this about a nose? While perhaps not nearly as glamorous as the beautiful eyes, the nose does serve two primary purpose: one of containing the sense of smell tissues and cells, and the other as an entry way, via the nasal passage and cavity, for air to be breathed in and out of the lungs, the respiratory system.
The sense of smell is accomplished in animals with a group of tissues and cells called the olfactory system. Animals vary greatly in the development of their olfactory system. The dog, for instance, has a highly defined system, much greater than in humans. This is why they are used to hunt for contraband or find the trail of a lost person. And that is why once lost and maybe a long ways from home, they may smell their way home with a highly developed olfactory system. They are really quite remarkable animals, and thank goodness for them.
Contained in the nose are olfactory receptors. These are highly developed nerve cells that are directly connected to the nervous system, and end up carrying stimuli to the brain for interpretation. At the outermost portion of these receptors are very fine cilia, a hairlike protrusion into the open cavity of the nostril. As air is breathed in, the various molecules suspended in the air pass through the nasal cavity and brush up against these cilia.
Well, the molecule of say, the newly fed flake of alfalfa hay, is embedded into the cilia. The cilia are bathed in mucous, and for odors to be embedded in the cilia, they must first be dissolved in this mucous blanket. The olfactory cell, of which these cilia are at one end, is stimulated by this molecule at the base of the cell, called the olfactory bulb. Deeply within the olfactory cell, the stimulated olfactory bulb excites the olfactory receptor neuron. Here then, the molecule of alfalfa odor is converted into an electrical impulse.
At the base of the olfactory cell, which is deeply embedded into the epithelial tissue of the nasal cavity, is a fiber called the afferent fiber. This fiber is a part of the central nervous system, and thus, is responsible for carrying electric stimuli to the brain. Of interest is this: just about all central nervous system cells are not able to regenerate, that is, once they are gone, the sense of stimuli is lost. Olfactory cells of the nasal cavity do regenerate, and in fact are replaced every one to six months in animals.
Another quite interesting aspect of smell in ruminants, is the bulls' ability to sense sex hormones in the female. Ruminants possess an accessory olfactory system in which very specific olfactory cells serve as receptor sites for the molecules that make up sex hormones. Thus, a cow in heat is readily detected because the air space around her contains the discharged molecules of sex, and the bull is readily able to interpret them as, well, you know where this is leading.
Another role the nose and nasal passage serve is for the entry of fresh air and moisture into the animal and the exiting of gasses and moisture from the lungs. The nasal cavity ends at the pharynx in the throat. Here, the nasal passage combines the two nostrils into one common passageway that is directly connected to the bronchial tube, and thus, to the lung cavity and tissue. When an animal breaths threw the nose, the air intake volume is moved into the bronchial tube. Only when an animal swallows food is the small bit of tissue called the epiglottis moved to direct food into the esophagus and then the stomach or rumen. The epiglottis remains closed by the action of sphincter muscle, part of the throat musculature that ensures that breathing takes top priority; thus we must consciously think about swallowing so that food does not end up in the lungs.
One very good purpose of breathing threw the nose is the warming action of moving the air volume over the warm mucous in the nostrils so it does not shock the lung tissue. Extreme cold air breathed threw the mouth will constrict blood vessels in the lung tissue, thus reducing the gaseous interchange so crucial to life.....oxygen that oxygenates blood and the release of carbon dioxide that results from cellular oxidation as the body is maintained and work is done.
So too, the nostrils may serve as a filter. The small hairs and cilia will catch a fair bit of the larger particles that contaminate the air. Dust, smoke, pollen, and even microbes are caught here. We may chose to blow our nose to remove them, or in fact the great majority of this stuff is removed during the exhale cycle of respiration.
We have all seen a cow discharge a big volume of air and send feed or mucous all over.....this is a way of cleaning out her nasal cavity. Sometimes a sneeze does the same for us. Interesting to note here is that as respiration in any animal is increased, usually in response to flight or fight or exercise, the greater proportion of air volume is moved threw the mouth and less threw the nose.
Finally, the very part of the nose that rested in Harmon's hand, is a very tough and very strong (we call this kind of tissue cornified) epidermal tissue that is devoid of hair follicles. This part of the nose does contain tubular merocrine (partly secreting) glands that secrete salivary-like moisture. That is why a cows nose is usually moist.
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