Testicles are parts that are found on the bodies of male creatures. Male mammals, including humans, have two testicles, supported in a sac of skin below the penis called the scrotum. Along with the penis, testicles are called reproductory organs or "sex organs". Only males have testicles; females have ovaries. 

Testicles are a type of organ called glands. Like other glands, testicles make chemical substances, called hormones that keep the body working. Testicles also make sperm, which can join with ova to make new life.  

Most glands, like women's ovaries, are inside the body, but testicles are outside the main part of the body. This is because the testicles work better if they are cooler than the inside of the body.  In cold weather, testicles pull up nearer to the body to keep warm.

Description
The testicles are firm, oval-shaped glands. Most testicles  match in size, but some testicles are much larger or smaller than others. Normal sized testicles are between 14 cm³ to 35 cm³. 

Most men have one testicle that hangs lower than the other one. Scientists think this may be so that they do not hit against each other. It is usually, but not always, the left testicle that hangs lower.

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. When this muscle contracts (pulls tighter), the cord gets shorter, which pulls the testicles closer to the body. If the testicles are too warm, the cremasteric muscle relaxes (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 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.

Function 
Testicles have two important jobs in the body. They are part of two of the body's systems, the endocrine system which keeps the body working, and the reproductive system which makes new life.

Making hormones
Testicles are a type of organ called glands. (This makes them part of the body's endocrine system.) The human body has many types of glands. The job of glands within a body is to make chemical substances, and put them out into the body's system. A body has lots of different systems that keep it in good working order all the time, and that also provide for a body's special needs. Some of the body's special needs happen when a person gets angry, frightened or sick, or if the person wants to have sexual intercourse. At these times a body uses more of some types of chemical substance, so the glands that make them work harder. 

Testicles make several types of chemical substances. They are not simple chemicals, but are very complicated and very important to life. Making these substances is controlled by the pituitary gland which is a small gland in the brain.

One substance made by the testicles is a type of substance known as a hormone. It is the hormone testosterone. Testosterone is important in a male person's body, because it makes him grow into a man and feel like a man during puberty. (Women's bodies make some testosterone too, but they make more female hormone called estrogen.)

Making sperm
As well as being chemical-producing glands, testicles are gonads. (This makes them part of the body's reproductive system.) Apart from hormones, the other important substances made by the testicles are spermatozoa, which are generally just called sperm. (The word "sperm" is plural, so you do not add an "s" and say "sperms".) The sperm are tiny living cells which can join with another cell, called an "ovum" (or egg) inside a female, to start a new human life. Scientists call making sperm spermatogenesis. Boys begin making sperm when they start growing into men, at a time of life called puberty. 

Females do not have testicles. The glands in their bodies that do the same sort of jobs as testicles are called ovaries. They have two ovaries, but unlike testicles, ovaries are inside the body, on either side of the uterus. They release ova (or eggs) and female hormones. Ovaries and testicles are sometimes called "gonads".

Details of how testicles are constructed and how they work 
(See the picture, right)
 The testicles, inside the scrotum, are covered with a tough type of white protective skin called a membrane. 
 The inner part of the testicles have many small tubes called seminiferous tubules which are coiled (or twisted around). Sperm are made inside the seminiferous tubules. 
 The sperm pass through some small tubes called ducts into another part at the back of the testicle, called the epididymis, which is where the sperm cells finish maturing. 
 The sperm cells then travel along a tube called the vas deferens until they reach the urethra. 
 The urethra is the main tube in the penis and leads to the opening at the end called the urethral opening. The urethral opening is also the way that urine leaves the body from the bladder.
 When a man feels sexual desire, sperm cells go through the ejaculatory duct. They are combined with liquid called seminal fluid from a gland called the prostate. The prostate also has muscles that push the sperm and fluid into the urethra.
 During sexual intercourse, seminal fluid containing the sperm comes out through the urethral opening when a man ejaculates. 
 If the sperm from the testicles enters the vagina of a woman during sexual intercourse, it may pass into the uterus and join with a mature ovum (or egg) to begin a new baby.

Health issues

Size 
Size in animals

Studying animals can help in understanding things about people. In mammals, testicles can be very big or very small in relation to the size of the animal itself. The size has to do with the amount of sperm that the male animal needs to make. Some types of male animal generally only have one sexual partner at a time and are called monogamous. Some types of animals generally live in a herd or flock where one male will have many female sexual partners. These males are called polygamous. More sperm is needed by polygamous males than by monogamous males. Polygamous males generally have larger testicles than monogamous males. The testicles grow larger to make more sperm.

Size in men

Most men's testicles are from 14 cm³ to 35 cm³. This is a measure of volume. Sometimes Doctors need to measure a man's testicles to see if there is a problem. Doctors can measure the volume of testicles in two ways: 

 Orchidometer – This is a set of small objects that look like eggs. They are many different sizes. The doctor picks the one that is the size of the testicle. Doctors know what size the orchidometer is. Then they know the size of the testicle. 
 Measuring the size – with a ruler, calipers, or ultrasound. Then these lengths are used to calculate the volume.

Sometimes the size of a man's testicles changes.

Some reasons why testicles get smaller are: 
 If a man takes testosterone, the testicles do not have to work to make testosterone, so they become smaller.
 If a man's testicles are kept more warm than usual, they make less sperm. So if they are kept too warm, they get smaller.
 If testicles are injured(hurt) they can become smaller. 

Some reasons why testicles get bigger are: 

 If a man takes hormones from the pituitary gland, (called gonadotropins), it can make testicles bigger. Gonadtropins are the hormones that tell the testicles to make sperm and testosterone. 
 Some diseases cause testicles to swell.

Diseases 
The most important diseases of testicles are:
 Inflammation of the testicles. This is called orchitis
 Testicular cancer 
 Fluid around a testicle. This is called a hydrocele
 Inflammation of the epididymis. This is called epididymitis 
 Spermatic cord torsion. This is also called Testicular torsion. It is a medical emergency. It is when the cord that the testicle hangs from gets twisted. It can happen when running.
 Varicocele - swollen veins to the testes. Varicocele happens more on the left. 

Loss and injury 

Most boys are born with two testicles in the scrotum. The testicles have formed inside the baby's body, but moved into the scrotum before the birth. Sometimes one or both the testicles are still inside when the baby is born. Sometimes an operation is needed to fix this. 

If a testicle is injured, it is extremely painful. Sometimes testicles get crushed. Sometimes it is necessary for a doctor to remove one or both the testicles. This operation is called an orchidectomy. If a man loses a testicle, a doctor can put a testicular prosthesis (false testicle) into the scrotum. It looks and feels like a testicle. 

If a male loses both his testicles it's called castration. Because the man cannot produce sperm or testosterone, it has a big effect on the man's life. Because he cannot make sperm, he cannot have children. And because he cannot make the male hormone testosterone, he becomes less like a male. He may lose some of his muscles, and put on weight. A man who has lost his testicles may take testosterone medicine so that he does not have these changes. 

Until the 1700s in Europe, and more recently in some countries, there was a custom to castrate young male slaves and boys. Young slaves were castrated so that they could be used as guards over the wives of rich men. It meant that the wives would not fall in love with their guards. Sometimes young boys who had beautiful singing voices were castrated before puberty. This meant that their voices would stay high all their lives. These singing men were called castrati. 

Castrating animals 

It is very common to castrate (remove the testicles from) male domestic animals. Male dogs and cats that are castrated do not fight so much, and do not stray around looking for females to mate with. 

Male horses are usually castrated so that they become more calm and safer to ride. A castrated male horse is called a gelding. A male horse that is not castrated is a stallion. 

Male animals that are used for meat are generally castrated because it makes them fatter and also makes them quieter to keep because they do not fight each other. Most male cattle are castrated.

Other names for testicles 
Testicles can also be called "testes" or "gonads".

There are many slang words for testicles, like balls, nuts, bollocks, nads, crown jewels, testies, marbles, ding dangs, boy toys, nicnaks, plums,tattay (in Urdu) etc. Slang words are other names that are used for fun or to be vulgar (rude).

Related pages
 Cryptorchidism (cryptorchismus)
 Infertility
 List of homologues of the human reproductive system
 Orchidometer
 Spermatogenesis
 Sterilization (surgical procedure), vasectomy

References 

Anatomy of the male reproductive system
Glands