As a general rule, verb tenses within a sentence or a paragraph should be consistent. A shift in tense without reason distorts the sequence of events being described and will confuse your reader. For example, if you begin a description with a verb in the past tense, do not switch to a verb in the present tense.
Surgery made critical advances in the 19th century. The
discovery of anesthesia by Crawford Long in 1842 and W. T. G. Morton in 1846
gave doctors greater freedom to put their new anatomical
knowledge to practical use. After Louis Pasteur discovered
that microorganisms caused infections, Joseph Lister (1827-1912), a young
Scottish surgeon, realized that antiseptic surgical techniques
would allow surgeons to suture wounds and avoid infection.
Finally, Roentgen reports his discovery of X-rays (or
roentgenograms) in 1895, which is immediately put to practical
use in surgical diagnosis.
Surgery made critical advances in the 19th century. The
discovery of anesthesia by Crawford Long in 1842 and W. T. G. Morton in 1846
gave doctors greater freedom to put their new anatomical
knowledge to practical use. After Louis Pasteur discovered
that microorganisms caused infections, Joseph Lister (1827-1912), a young
Scottish surgeon, realized that antiseptic surgical techniques
would allow surgeons to suture wounds and avoid infection.
Finally, Roentgen reported his discovery of X-rays (or
roentgenograms) in 1895, which was immediately put to
practical use in surgical diagnosis.
--Linglei Xu, "Bone Talk," SciTech Magazine (modified)
See also the discussion of sequence of tenses.