Will the hard cell of a cow horn grow back?
Not sure what you mean here, but when a bovine is dehorned, the horns don't grow back, especially if they were cut at the root and not an inch away from the skull. If the horns were cut only an inch away from the skull, they still will grow, but not normally like they would if they were not cut off.
Not if the dehorning process was properly done! Horns must be cut right to the base of the skull, not an inch or a sixteenth of an inch to spare, in order for them to not grow back. If a little bit of horn is left (even a tiny little bit), it will grow back, for sure. But, the horn will not grow back as normally as it would if it were not cut off…
Nerve cells can grow axons up to six feet in length. They are how ever the biggest. The Human egg cell is the biggest. The human ovum is typically around 120um and yes, it is visible to the human eye. The Anterior Horn Cell of the spine however, is around 135um thus making it the largest in circumference.
Cows do have feet, but they are known as hooves. They are hard, like a horse's hoof, but are cloven, meaning that they are divided... like having two "toes." A cow's hoof has a hard, rubbery sole and a hard, outer covering of material like horn. They grow constantly and need to be worn off or trimmed.
Poachers kill rhinoceroses, whether they have one or two horns, because in parts of Asia, especially China, rhinoceros horn is highly valued as a medicine. However, rhinoceros horn can be harvested from live rhinoceroses without harming them, and it will then grow back. Unfortunately, it is not currently legal to sell farmed rhinoceros horn. If farmed rhinoceros horn were legal, poachers would probably be put out of business.
The concepts are so similar traced back to their roots, that it would be hard to tell for sure, since the most basic difference is the mouthpiece: Cup shaped in a trumpet, funnel shaped in a horn. As best as I know music history (my degree was in music) I would offer that the trumpet concept came first - it is easily traced back to the Romans. The French Horn orchestral instrument evolved out of…