516 Animal Biology 



afian follicle cells remaining in the ovary organize themselves into a yel- 

 lowish ductless gland called the corpus luteum (L. luteolus, yellowish), 

 which is described in a summary of endocrine glands earlier in this chap- 

 ter. If the ovum is not fertilized, the corpus luteum degenerates. The 

 human ovum is very small (0.15 mm. in diameter) because of a minimum 

 of food (yolk). Consequently, the developing embryo must have nour- 

 ishment from the mother. The ovum while in the upper part of the 

 Fallopian tube produces a small, nonfertilizable polar body (polocyte) 

 (Fig. 351) and a second polar body after fertilization. Ovulation occurs 

 at regular periodic intervals and the series of interrelated phenomena, 

 including the preparation of the uterus for the implantation of the fer- 

 tilized ovum, is called the estrous cycle. The estrous cycle in the human 

 female occurs more or less within twenty-eight days but may be altered 

 by mental shocks, psychic disturbances, worry, physical illness, climatic 

 changes, etc. If fertilization does not occur, the superficial mucous layer 

 of the uterus is shed and accompanied by rupturing of blood vessels (hem- 

 orrhage). This ends in menstruation in which tissues and blood leave 

 the uterus through the vagina. 



A study of the development of a human being will reveal (as in many 

 other animals) numerous vestigial (rudimentary) organs which are re- 

 duced in size and are without appreciable use at present, although they 

 may have been larger and functional in the past. Many of them seem 

 to be in the process of disappearing, having served their period of useful- 

 ness. Over one hundred vestis:ial structures and ors^ans occur in the 

 human body, among the more common being ( 1 ) vermiform appendix, 

 (2) special muscles to move the ears, (3) lobe at the bottom of the ear, 

 (4) point ("Darwin's point") at inner curled ridge of the upper margin 

 of the ear, (5) whitish nictitating membrane (third eyelid) in inner angle 

 of the eye, (6) third molars ("wisdom teeth"), (7) hair on body, (8) 

 special patterns or arrangements of hair on various parts of the body, 

 (9) small muscles to erect body hair, (10) mammary glands in male, 

 (11) segmented muscles of abdominal wall, (12) caudal (tail) vertebrae 

 called the coccyx. Many of the vestigial structures of man have their 

 homologous structures in related lower organisms in which cases they 

 are functional. This is evidence that organisms have changed struc- 

 turally and functionally; in other words, have evolved. 



XI. DISEASES OF MAN 



Disease may be defined as an abnormal or pathologic condition of any 

 part of the body or mind. Diseases may be classified as (1) infectious, or 



