The testicles, known medically as testes (singular testis), are the male generative glands in animals. Male mammals have two testicles, which are often contained within an extension of the abdomen called the scrotum.

Function

Like the ovaries (to which they are homologous), 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)
• producing male sex hormones, of which testosterone is the best known

Both functions of the testicle, sperm-forming and endocrine, are under control of gonadotropic hormones produced by the anterior pituitary:

• luteinizing hormone (LH)
• follicle-stimulating hormone (FSH)

Structure

Under a tough fibrous shell, the tunica albuginea, the testis contains very fine coiled tubes called the seminiferous tubules. The tubes are lined with a layer of cells that, from puberty into old-age, produce sperm cells. The seminiferous tubules lead to the epididymis, where newly created sperm cells mature, and then into vas deferens (also called the ductus deferens) which opens into the urethra. Upon any sufficient sexual arousal, the sperm cells move through the ejaculatory duct and into the prostatic urethra, where the prostate, through muscular contractions, ejaculates the sperm, mixed with other fluids, out through the penis. (The genital anatomy described here, along with the neuroanatomy and hormonal systems that enable it to perform ejaculation, have as primary evolutionary functions the impregnation of a fertile female of the same species (or a sufficiently close one), via sexual intercourse with her.)

Transverse section through the left side of the scrotum and the left testis. The sac of the tunica vaginalis is represented in a distended condition.

Between the seminiferous tubules are special cells called interstitial cells (Leydig cells) where testosterone and other androgens are formed.

Testicluar size

Testicular size in relation to body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to be larger when the species is more likely to be polygamous than monogamous. Production of testicular output is also larger in the polygamous animal, possibly a spermatogenic competition for survival.

In normal adult human males, testicular size ranges from the lower end of around 14 cm³ to the upper end larger than 35 cm³. Measurement in the living adult is done in two basic ways: (1) comparing the testicle with ellipsoids of known sizes (orchidometer), or (2) measuring the length, depth and width with a ruler, a pair of calipers or ultrasound imaging. The volume is then calculated, e.g. using the formula for ellipsoids: π/6 × length × width².

To some extent it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g. higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Similarly, stimulation of testicular functions via gonadotrophic-like hormones may enlarge their size.

Health issues

The testicles are well-known to be very sensitive to impact and injury. This has been a rich source of humor for jokes and comedic routines. Slang terms for testicles, like "balls" or the Spanish "cojones" are often used in everyday speech to denote courage or audacity, as in "He has balls to do that."

The most important diseases of testicles are:

• inflammation of the testicles, called orchitis
• testicular cancer and other neoplasms
• accumulation of clear fluid around a testicle, called hydrocele testis
• inflammation of the epididymis, called epididymitis
• spermatic cord torsion also called testicular torsion
• varicocele [1] - swollen vein to the testes, usually affecting the left testicle

The removal of one or both testicles is termed

• orchidectomy, in medicine (where orchiectomy and orchectomy are synonymous), and
• castration in general use, especially when done for the benefit of others than the subject, for example, to produce a high-voiced castrato from the castration of a pre-pubescent boy

At least for humans, testicular prostheses are available to mimic the appearance and feel of one or both testicles, when absent as from injury or medical treatment.

See also

• cryptorchidism (cryptorchismus)
• infertility
• List of homologues of the human reproductive system
• orchidometer
• spermatogenesis
• sterilization (surgical procedure), vasectomy
• Image:Human penis flaccid.jpg - shows right-hand side of scrotum with right testicle.