220 PHYSIOLOGY AT THE FARM. 



membranes. In the spheroidal epithelium the cells retain 

 nearly their primitive form, or so changed that they have a 

 polyhedral aspect. The chief seat of this description of epi- 

 thelium is in the urinary passages, with the exception of the 

 urethra, in which the epithelium is columnar. 



In ciliated epithelium, the particles, which are generally 

 columnar, bear at their free extremities little hair-like processes, 

 which are agitated incessantly during life, and for some time 

 after death, with a lashing or vibrating motion. These minute 

 and delicate moving organs are named cilia, from the Latin 

 word signifying eyelashes. In the human body and mammi- 

 ferous animals in general, ciliated epithelium occurs in the 

 following parts : on the mucous membrane of the air-passages 

 and its prolongations ; on the mucous lining of the uterus ; on 

 the parietes of the ventricles of the brain. 



The nature of ciliary movement is still involved in much 

 obscurity. 



The chemical composition of epithelium is still somewhat 

 uncertain. 



The epidermis of the integuments appears to be similar to 

 horny matter in composition the chief constituent being a 

 substance intermediate between the albuminoid and gela- 

 tigenous components of the body, joined to a considerable 

 proportion of sulphur. 



Pigment. See Pigment- Cells, p. 217. 



Adipose Tissue or Fat. In the most healthy state of the 

 body there is present a considerable amount of fat. It is found 

 in the blood and in the chyle, and, though in much more 

 sparing quantity, in the lymph. It exists also in the products 

 of secretion, as in the milk, the sebaceous matter of the skin, 

 the cerumen of the ears, the bile, and, moreover, in the forma- 

 tion of the corpus luteum in the ovaries. But the greater part 

 of the fat is contained in small cells or vesicles spread over the 



