The colliculus seminalis and surrounding duct openings
Keywords: colliculus, gelding, anatomy, equine
The horse shoe-shaped structure seen below is the border of the colliculus seminalis in a gelding. It is situated on the dorsal wall of the urethra just caudal (distal) to the bladder. The orifice towards the bottom of the image is the opening to the bladder (b) and the curved slit-like openings (ed) that are contained within the caudal part of the curved ridge are the so-called “ejaculatory ducts”. Later in this entry, the author suggests an alternate name for the ejaculatory ducts.
Image size: 901 x 953px
The colliculus is often shown upside down in text books. This is because the endoscopes used to capture those images have fiber-optic systems with hand controls (air/flush) on top of the handles. As those endoscopes curve over the ischial arch, structures situated dorsally in the urethra appear at the bottom of the fiber-optic image; a source of some confusion to the uninitiated. Unfortunately authors seldom think to simply invert these images when they are published! Fortunately, with video-endoscopy, an operator can rotate the endoscope and orient structures correctly.
| In vintage medical medical terminology the colliculus was referred to as the verummontanum, the Latin term for a mountain ridge. Nowadays, because the term colliculus is used, it is tempting to deduce that the colliculis as a place where semen collects but this is not correct. The term is actually derived from Latin, where “colliculus” is the diminutive form of “collis” or hill. Those who speak any of the romance languages and who also hold a fascination for etymology (and geography) will know that the name for a hill is variously "colline", "collina", "colina" etc |
In the image below from a mature stallion, one can see the anabolic effect of androgens on the development of the colliculus seminalis and various duct openings. From top to bottom in this image, as the endoscope is advanced, the two rows of openings (left and right) of the bulbourethral glands can be seen. More cranially, within the horse-shoe shape of colliculus itself, the ejaculatory ducts are visible, and on the sides of the colliculus, several openings of the prostate glands.
Image size: 1143 x 1006 px (click on image for detail)
This image shows a ventral view (a view of the dorsal aspect of the urethra) of these dissected structures in the same stallion:
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In this view, the author has taken the liberty of using the term common ejaculatory ducts instead of ejaculatory ducts because these short ducts contain both the openings of the ducts of the vesicular glands and the ductus deferentia. In this specimen, where the left common ejaculatory duct has been opened, one finds the openings of the ductus deferentia medial to those of the vesicular glands.
With gentle manipulation, a 9mm endoscope can pass through the openings of the vesicular glands.
| A fascinating reference from 1884 (The Anatomy of the Horse: A Dissection Guide by Sir John McFadyean) states that the ducts are: …sufficiently large to permit the tip of a little finger being insinuated into them. “Insinuated” in this sense means to maneuver by subtle manipulation. The author doubts that this term will ever become indispensable in the lexicon of a theriogenologist!. |
Between the openings of the common ejaculatory ducts one may a small opening may be present on the midline. It was not visible in this specimen. It is a fused remnant of the two tubes of the mullerian system that have been suppressed by anti-mullerian hormone from Sertoli cells in the testicles. Therefore it is akin to the uterus in females. For that reason it is referred to as the uterus masculinus. In some texts however it is referred to as the utriculus masculinus or even the utriculus prostaticus (because it is found adjacent to the prostate gland). The latter term is inaccurate because “utriculus” actually refers to a sac in Latin, while “uterus” obviously refers to the female organ. As reviewed by Pozor et al., the uterus masculinus may become cystic and occlude the common ejaculatory ducts, interfering with ejaculation.
The following image relates the openings of glands within the common ejaculatory duct and the glands themselves. Because the dissected tract has a somewhat homogeneous color, structures have been color coded to make them more visible.
Image size: 2500 x 1741px
Reference:
Pozor, M.A. et al. 2011. Midline cysts of the colliculus seminalis causing ejaculatory problems in stallions. J. Vet Equine Vet. Sci. 31: 722-721
