Adoctor is exploring some tissues with a blunt probe. she notices that the tissue is white and fatty and seems to be concentrated within the spinal cord. she is observing
a. gray matter
b. myelin sheaths

The correct answer is option B, that is, myelin sheaths.

Explanation:

Myelin refers to an insulating layer, or a sheath, which get produced surrounding the nerves, comprising those in the spinal cord and the brain. It is formed of fatty substances and protein. This myelin sheath permits electrical impulses to conduct briskly and effectively between the nerve cells.  

The neuron’s myelin sheath comprises the cells containing fat, which shield the axon from electrical activity. The insulation functions to enhance the rate of conduction of signals. The myelin sheath provides the whitish look to the brain's white matter.  

humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. the only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome.