Human testis. Coloured ultrasound scan of a healthy testis (orange) of a 59 year old man. The testes (testicles) are the sites of sperm production in the male body. They are carried outside the body in the scrotal sac, seen here bordering the testis. The testicles are carried outside the body because the optimum temperature for sperm development is slightly cooler than normal body temperature. Ultrasound scanning uses the reflections of high- frequency sound waves to produce images of internal structures. It is especially useful for looking at cavities, fluid- filled spaces and tumours, and is a valuable diagnostic tool.