GLOSSARY. 159 



Vis a ter'go (L. force behind). A pushing force by which the 



flow of a fluid is assisted, as in the ascent of the sap in 



plants, where the absorption of moisture by the spongioles 



of the rootlets assists in pushing on the sap in its upward 



course. 

 Visnervo'sa (L. nervous force). A term applied to signify the 



property of nerves whereby they conduct impressions. 

 Vis'cera (L. the entrails). Applied to all the organs contained 



within the body. 

 Vis'cus (L. viscus, the inside of the body). Any internal organ 



of the body. 

 Vitella'rium (L. vitellus, yolk of an egg). A gland which in 



some Turbellaria is connected with the female generative 



organs, and secretes a special yolk-like material. 

 Vltellfgenous cells (L. vitellus, yolk; Gr. gennao, I produce). 



Cells, secreted by the ovarioles of some insects, which 



probably supply nutriment to the growing ova. 

 Vi'telline duct (L. vttellus, yolk). The duct by which in early 



foetal life the yolk sac communicates with the embryonic 



sac of the ovum. 

 Vi'tellinie mem'brane (L. v^tellus, yolk). The membrane which 



encloses the yolk of the ovum. 

 Vitellus (L. yolk of an egg). The general contents of the 



ovum. 

 Vit'reous (L. vitrum, glass). The name given to the humour 



which occupies the posterior chamber of the eye, on 



account of its glassy appearance. 

 Vivi'parous (L. vivo, I live; pario, I bring forth). Applied to 



those animals which bring forth their young alive. 

 Volar artery (L. vola, the palm of the hand). A branch of 



the radial artery distributed to the hand. 

 Vo'mer (L. a ploughshare). A bone of the skull, so named 



because of its fancied resemblance to a ploughshare. 

 VulVa. A general term for all the external parts of the female 



generative organs. 



W. 



Willis, circle of. Anastomosis of the branches of the 

 vertebral and internal carotid arteries in the skull. 



Wolffian bodies. Temporary organs in the foetus from which 

 the urinary and reproductive organs are developed. 



