Dictionary

ascending Going upward. For example, the ascending aorta is the portion of the aorta that ascends, going upward as it leaves the heart to form the beginning of the arch of the aorta.

descending Going down. For example, the descending aorta is the portion of the aorta that descends, going downward from the top of the arch of the aorta.

external Situated on the outside.

extra Prefix indicating outside of.

yolk bone See zygoma.

yolk sac The membrane outside the human embryo. The yolk sac is connected by a tube, the yolk stalk or omphalomesenteric duct, through the umbilical opening to the embryo’s midgut. The yolk sac serves as an early site for the formation of blood, and in time it is incorporated into the primitive gut of the…

yolk stalk See omphalomesenteric duct.

youth The time between childhood and maturity.

Z chromosome A sex chromosome in certain animals, such as chickens, turkeys, and moths. In humans, males are XY and females are XX, but in animals with a Z chromosome, males are ZZ and females are WZ.

zebra In medicine, a very unlikely diagnostic possibility. It comes from an old saying used in teaching medical students about how to think logically in regard to the differential diagnosis: “When you hear hoof beats, think of horses, not zebras.” For example, when someone develops a mild transient cough, a virus infection is the most…

Zellweger syndrome One of a group of related diseases called peroxisome biogenesis disorders (PBD), inherited conditions that damage myelin in the brain and also affect the metabolism of particular substances. Features include enlargement of the liver, high levels of iron and copper in the blood, and vision disturbances. It can be caused by mutations in…

Zenker diverticulum The most common type of outpouching in the esophagus, due to increased pressure causing the mucous membrane of the esophagus to herniate through a defect in the wall of esophagus. Zenker diverticuli are usually located in the posterior hypopharyngeal wall. Small Zenker diverticuli may not cause symptoms but larger ones may collect food…