Final answer:
The dartos muscle is the layer of smooth muscle in the skin of the scrotal sac, forming the subcutaneous muscle layer of the scrotum and helping regulate testicular temperature.
Step-by-step explanation:
The layer of smooth muscle in the skin of the scrotal sac is known as the dartos muscle. This muscle layer makes up the subcutaneous layer of the scrotum, and it also continues internally to form the scrotal septum, dividing the scrotum into two compartments. Each compartment houses one testis. In addition, the cremaster muscles, which originate from the internal oblique muscle of the abdominal wall, cover the testes like a muscular net. These muscles work in tandem to regulate the temperature of the testes by moving them closer to or farther from the body, depending on environmental temperature changes.