Provide definitions/explanations for the following terms:
$(1)$ Column of Bertini
$(2)$ Excretory unit (nephron)

(N/A) $(1)$ The renal cortex extends in between the medullary pyramids as renal columns,which are known as the columns of Bertini.
$(2)$ The nephron is the structural and functional unit of the kidney,responsible for the filtration of blood and the formation of urine.