Appendix 863 



Ventral (ven' tral) (L. venter, belly), lower or belly side. 



Ventricle (ven'trikal) (L. ventriculus, little belly), lower heavier chamber of 



the heart from which blood is pumped out. 

 Vermiform appendix (ver' mi form) (L. vermis, worm; forma, form), slender 



appendage of the large intestine where it joins the small intestine. 

 Vertebrate (vur' te brate) (L. vertebratus, backbone), animal having a vertebral 



column. 

 Vestigial (ves -tij' i al) (L. vestigium, trace), rudimentary part of an organism 



no longer functionally useful. 

 Villus (plural villi) (vil' us) (L. villus, hair), minute projection of small intestine 



to increase absorption. 

 Virus (vi' rus) (L. virus, poison), living ultramicroscopic, virulent cause of certain 



plant or animal diseases. 

 Viscera (vis' era) (L. viscera, internal organs), organs within a body cavity. 

 Viscosity (vis -cos' i ti) (L. viscosus, y'lscous) , tendency of certain liquids not to 



flow easily due to internal friction (adherence of liquid particles to each 



other). 

 Vitalism (vi' tal izm) (L. vita, life), the doctrine which attributes at least some 



of the living phenomena to an interplay of nonmaterial forces other than 



those prevailing in the lifeless world (contrast with Mechanistic view). 

 Vitamin (vi'tamin) (L. vita, life; amin, a chemical radicle, NH2), substance 



which is essential for the proper metabolism and regulation of body proc- 

 esses; they were named vitamins because they were thought originally to 



contain an amine radicle, which is incorrect. 

 Viviparous (vi -vip' a rus) (L. vivus, alive; parere, to bear), development of the 



embryo within the mother's body and the subsequent birth of a living 



young organism. 

 Vomer (vo' mer) (L. vomer, ploughshare), bony partition in the nose. 



W 



Warm-blooded, animals whose blood retains a rather constant temperature re- 

 gardless of external temperatures, as birds and mammals (contrast with 

 Cold-blooded). 



White blood corpuscle, colorless blood cell; also called leucocyte. 



Wolffian duct (after German anatomist, Wolff), the forerunner of the male vas 

 deferens in vertebrates. 



Working hypothesis (hi -poth' e sis) (Gr. hypo, under; tithemi, place or consider- 

 ation), a basic assumption to guide the study of a problem or subject and 

 to be proved or disproved by the data accumulated. 



X 



Xanthophyll (zan' tho fil) (Gr. xanthos, yellow; phyllon, leaf), yellow-orange pig- 

 ment of certain higher plants, especially leaves. 



X chromosome, a chromosome associated with sex of many organisms. 



Xerophyte (ze' ro fite) (Gr. zeros, dry; phyton, plant), plant adapted to dry 

 conditions. 



Xylem (zi' lem) (Gr. xylem, wood), woody, water-conducting portion of a fibro- 

 vascular bundle. 



