The species concept is practical for identification of unfamiliar species and has been used to describe most of the 1.8 million known species on earth.

Bundles of axons in the central nervous system are called a "tract"

Explanation:

Axons(nerve fiber) consist of long slender projection of neurons or nerve cells that conduct and transmit impulses away from the neuron or nerve cell body. Axons are surrounded by endoneurium layer which contain protein fluid that surround each axon.

Bundles of axon in the CNS is known as tract. We have ascending tracts and descending tracts. The ascending tracts function are to conduct and carry impulses along the spinal cord to the brain while the descending tracts transmit and carry the impulses from the brain to lower regions (downward) of the body. Tracts are identified by their origin (where the bundle start) and also the end/termination of axon bundle.

The largest tracts are fasciculi gracilis and cuneatus which are known as ascending tracts.