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seminal vesicle[′sem·ən·əl ′ves·i·kəl]
in male salientians and birds and in some male fish and mammals (including man), a glandular widening or protrusion of the terminal portion of the seminal duct that releases an alkaline mucosal secretion needed to nourish and mobilize spermatozoa. In most animals the secretion of the seminal vesicles is a component of semen that serves to liquefy sperm and, in some animals, promotes the formation of sper-matophores from sperm.
Seminal vesicles are paired or unpaired. They are formed during embryonic development from various portions of the deferent ducts and may be simple widenings or sacculated processes of the ducts. In some animals seminal vesicles are independent structures united to the deferent ducts only by a narrow canal. Seminal vesicles are especially developed in some rodents and insectivores; they are absent in monotremes, marsupials and many predatory animals.
In man, the seminal vesicles form a paired organ 4–5 cm long, 2 cm wide, and 1.0–1.5 cm thick. They are located in connective tissue between the posteroinferior surface of the urinary bladder and the rectum, adjacent to the base of the prostate. The vesicles are convoluted tubes with numerous recesses. The vesicular wall consists of an external connective-tissue membrane, a weakly developed muscular membrane, a submucosal layer, and a mucous membrane with columnar or cubical epithelium. The excretory duct of the seminal vesicles is connected to the terminal portion of the seminal canal, forming the ejaculatory duct.