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.
In mammals the testes are located outside of the body as they are suspended by the spermatic cord and within the scrotum. This is due to the fact that spermatogenesis is more efficient at a temperature somewhat less than the core body temperature of 37 degrees Celsius (99 degrees Fahrenheit). The cremasteric muscle is part of the spermatic cord. When this muscle contracts the cord is shortened and the testicle is moved closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature. When the temperature needs to be lowered, the cremasteric muscle relaxes and the testicle is lowered away from the warm body and are able to cool. This phenomenon is known as the cremasteric reflex. It also occurs in response to stress (the testicles rise up toward the body in an evolutionary effort to protect them in a fight) and they also contract during orgasm.
It is normal for one testis to hang lower than the other (usually the left). This is primarily due to differences in the vascular anatomical structure on the right and left sides. It is thought that this is another evolutionary development which protects each testis from bouncing off the other.
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)
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.)
Between the seminiferous tubules are special cells called interstitial cells (Leydig cells) where testosterone and other androgens are formed.
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 Ã— length Ã— widthÂ². Usually right and left testicles have about the same size, but not exactly the same size.
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.
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
- accumulation of clear fluid around a testicle, called hydrocele testis
- testicular cancer
- inflammation of the epididymis, called epididymitis
- spermatic cord torsion also called testicular torsion
- varicocele  - 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.
- WikiSaurus:testicles — the WikiSaurus list of synonyms and slang words for testicles in many languages
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