8 ANATOMY OF THE RABBIT 



development of bones is considered in section II, B. A cross 

 section of the compact shaft of a dried long bone which has been 

 prepared for microscopic study shows an outer layer of circum- 

 ferential lamellae which Hes next to the periosteum and a 

 similar layer which lies next to the hollow, or narrow cavity, 

 of the bone. Both of these layers lie parallel to the internal and 

 external surfaces of the shaft, and thus their lacunae appear 

 in rows rather than in concentric (Haversian, Haver's) arrange- 

 ment, as is the case in the tissue lying between these two areas. 

 The interstitial (ground) lamellae are irregularly disposed plates 

 which, for the most part, fill the areas between the concentric 

 lamellae of the Haversian systems. In each of these cases the 

 lamellate appearance is due largely to the arrangement of the 

 cavities (lacunae) which were occupied by the cells while 

 the bone was alive. The Haversian (Haver's) system is essen- 

 tially a group of two or more concentric lamellae, alternating 

 with concentric rings of lacunae arranged around a central 

 cavity. Haversian canal. 



Haversian canals enclose blood and lymph vessels and thread- 

 like processes of marrow tissue. Lacunae are ovoid spaces, 

 "about 20 ji long, io^jl wide, and 6/x thick" (Addison), lying 

 between the rings of lamellae, and from which minute canals, 

 canaliculi, radiate to join other lacunae so that in life communi- 

 cation was established between all the cells of a Haversian 

 system. 



B. EPITHELIAL TISSUE 



EpitheUal tissues are primarily covering or protecting tissues, 

 such as the epidermis, the lining of the digestive tract and body 

 cavity; in short, the lining of all natural tubes and cavities in 

 the body is epithelial tissue. The predominating form of the 

 component cells of the various kinds of epithelial tissue are of 

 two chief groups, or types : Squamous and columnar. Each of 

 these two groups is again divided into simple and stratified epi- 

 thelium, depending upon whether it is composed of a single layer 



