Boys can see more easily their sex organs developing than girls because these are outside the body. First the testicles get bigger at around 12 to 14 years old and the penis gets bigger a year later.
Males, human and other mammals, usually have two testicles in their scrotum. These are firm, oval-shaped glands. Most men have testicles that match in size, but some men's testicles are much larger or smaller than other men's. It is normal for an adult man to have testicles that are between 14 cm³ to 35 cm³ in size.
Nearly all men have one testicle that hangs lower than the other one. Scientists think this may be so that they don't hit against each other. It can be either the left or the right that hangs lower, but in 85% of men, it is the left.
The testicles hang in the scrotum by the spermatic cord. They are outside the body because they need to be cooler than the temperature inside the body, in order to make sperm. If a man's testicles get cold, they are automatically pulled closer to the body. The spermatic cord is tightened by the cremasteric muscle. If the testicles are too warm, the cremasteric muscle relaxes or gets longer. This lowers the testicles to keep them cooler. This is how the testicles are kept at the right temperature. This is called the cremasteric reflex. (A reflex is something your body automatically does without you thinking about it.)
There are other uses for the cremasteric reflex apart from making the testicles at the right temperature. Stress can also make the cremasteric reflex work. If a male human or mammal gets into a fight, then its testicles are in danger. They automatically pull up close to the body. The cremasteric reflex also happens during sexual intercourse.
Sometimes the size of a man's testicles changes. The testicles get smaller if a man takes testosterone. The testicles do not have to work to make testosterone, so they become smaller. Meanwhile, if a man's testicles are kept more warm than usual, they make less sperm and they get smaller. Steroids taken for muscle enhancement often have the undesired side effect of testicular shrinkage. Also, if the testicles are injured or hurt, they can become smaller.
On the other hand, the testicles get bigger if a man takes hormones from the pituitary gland (called gonadotropins). Gonadtropins are the hormones that tell the testicles to make sperm and testosterone. Moreover, some diseases cause testicles to swell.
Sometimes, the testicles stay inside the lower abdominal tract or what is called undescended testes. Surgery can correct and put the testicles in the scrotum.
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Like the ovaries of the female, testicles are components of both the reproductive system (being gonads) and the endocrine system (being endocrine glands). The respective functions of the testicles are: producing sperm (spermatozoa) and producing male sex hormones, of which testosterone is the best-known.
The testicles are well-known to be very sensitive to impact and injury. Blue balls is a slang term for a temporary fluid congestion in the testicles and prostate region caused by prolonged sexual arousal.
The most prominent diseases of testicles are: testicular cancer and other neoplasms; swelling of a testicle, caused by hydrocele testis; inflammation of the testicles, called orchitis; inflammation of the epididymis, called epididymitis; spermatic cord torsion also called testicular torsion; varicocele or swollen vein from the testes, usually affecting the left testicle; and anorchidism is the absence of one or both testicles.
The removal of one or both testicles is termed as orchidectomy, in medicine and castration in general use, especially when done as punishment or torture, or as a catch-all term for orchidectomy in a veterinary context. On the other hand, it is called gelding specifically in the equine sense.