The descent of the testis is a developmental process. In the intrauterine life the testes develop on the posterior abdominal wall, at the level of the upper lumbar segments. From there they move down into the scrotum (a skin sac at the perineal region) by protruding all the layers of the abdominal wall in front of themselves. Thus, the coverings of the testis (and of the spermatic cord) correspond with the individual layers of the abdominal wall. (Figure 1.)
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Figure 1.: Coverings of the testis. The layers of the abdominal wall correspond to the coverings of the testes. At the left-hand side of the image a patent processus vaginalis is visible (perinatal conditions); the right-hand side part of the figure shows only the remnants of the processus vaginalis.
The scrotum includes a double cavity, since there is a midline separation (scrotal septum) which divides the scrotum into left and right compartments. In the scrotum the testes are located at their ideal temperature, being 2-5 degrees lower than that of the abdominal cavity. The lower temperature is supported by the position (outside the abdominal cavity), also by the well controllable heat transfer of the scrotum (tunica dartos) and by the counter-current cooling system of the testicular artery (provided by the pampiniform plexus). The higher temperature of the environment (e.g. spending a longer time in a hot bath) has adverse affect for the testicular spermatogenetic function, may cause temporary infertility.
III./1.5.2.: The coverings of the testis from outside to inside
III./1.5.2.1.:The scrotum
III./1.5.2.1.1.: The skin of the scrotum
This is the continuation of the skin of the abdominal wall. It is a thin, heavily pigmented, hairy skin, containing sweat and sebaceous glands (the secretion of the latter has a characteristic odor). The nerve endings of the scrotum are sensible to mechanical stimuli and to changes of the temperature. There is no subcutaneous adipose tissue here (the tunica dartos is found instead of it). In the midline a cordlike line, the raphe scroti is clearly visible.
The scrotum gets its blood supply from external pudendal artery (branch of the femoral artery), also from scrotal rami (of the internal pudendal artery) and the cremasteric branch (of the inferior epigastric artery). There are dense vascular networks making opportunity for heat dissipation. The scrotal lymph vessels drain into the superficial inguinal lymph nodes. Innervation: ilioinguinal nerve (anterior scrotal branches), genital branch of the genitofemoral nerve, pudendal nerve (from its perineal branches the posterior scrotal nerves), posterior femoral cutaneus nerve (perineal branches).
III./1.5.2.1.2.: Tunica dartos (dartos fascia)
It is the continuation of the abdominal subcutaneous connective tissue and the superficial abdominal fascia. Cold provokes the contraction of its smooth muscle (by controlling the heat dissipation is involved in the temperature regulation of the testis). Tunica dartos forms the scrotal septum also. This midline septum divides the scrotum into left and right compartments; the testes located in them have their own, separated coverings.
III./1.5.2.2.: Own (separated) coverings of the testis
III./1.5.2.2.1.: Cremasteric fascia (or external spermatic fascia)
This fascia is the continuation of the aponeurosis of the external abdominal oblique muscle.
III./1.5.2.2.2.: Cremaster muscle
Skeletal muscle, being the continuation of the next two muscular layers of the abdominal wall: the internal abdominal oblique and transversus abdominis muscles. The cremaster reflex is its nociceptive (or withdrawal) reflex: scratching the medial side of the thigh skin causes -via the L2-L4 spinal segments- contraction of cremaster muscle, thus elevation of the ipsilateral testis. The absence of this reflex can be pathognomonic in case of intraabdominal inflammation (e.g. in appendicitis).
Some animals (e.g. rodents) are able to pull their testes back into the abdominal cavity with the help of their well developed cremaster muscle (retractor testis muscle).
III./1.5.2.2.3.: Spermatic fascia (or internal spermatic fascia)
This is the continuation of the transversalis fascia.
III./1.5.2.2.4.: Tunica vaginalis testis
The peritoneal covering of the testis, develops from the processus vaginalis testis. Its parietal layer is fused with the spermatic fascia, while its visceral layer is tightly attached to the tunica albuginea.
