46 HISTOLOGY 



In the following pages the fundamental tissues will be considered in 

 turn. In connection with them, certain organs will be examined. An 

 1 organ is a more or less independent portion of the body, having a connect- 

 ive tissue framework, and a special blood, lymph, and nerve supply, 

 in addition to its characteristic essential cells. The essential cellular 

 substance of an organ, in distinction from the accessory tissues, is often 

 called its parenchyma; the accessory supporting tissues constitute the 

 stroma (Gr. arrpupa, bed), in which the parenchyma is imbedded. 



Such structures as the pancreas and liver are obviously organs. An 

 individual muscle or a particular bone, which has a connective tissue 

 covering or framework, and a supply of vessels and nerves, besides its 

 essential substance, may also be regarded as an organ. The organs which 

 are of widespread occurrence, such as the bones, muscles, tendons and 

 large vessels, will be described with the tissues. The more complex 

 organs are reserved for a later section, entitled "Special Histology." 



EPITHELIUM. 



The Dutch anatomist, Frederik Ruysch, recognized that the covering 

 of the margin of the lips is not identical with the epidermis. "There- 

 fore," he wrote, "I shall call that covering the epithelis, or papillary in- 

 tegument of the lips" (Thesaurus anat. Ill, 1703, No. 23, p. 26). It is 

 an unfortunate name (CTTI, upon O^Xrj, Latin papilla, the nipple) since it 

 does not refer to the layer upon the nipple, but to that which covers a 

 great number of nipple-like elevations of the underlying tissue. Such 

 elevations or papilla are indeed abundant in the lips, but they occur also 

 under the epidermis. Ruysch substituted epithelia for epithelis in other 

 sections of his work, and Haller, writing some years later, used the neuter 

 epithelium, so that epithelia thus became a plural. 



As the term epithelium is now used, it includes the epidermis, and the 

 lining of the various internal tubes and cavities. It has been defined as 

 a layer of closely compacted cells, covering an external or internal surface 

 of the body, having one of its surfaces therefore free, and the other rest- 

 ing on underlying tissue. But the term is also correctly applied to solid 

 outgrowths from such layers, either in the form of cords or masses of 

 cells. Usually these outgrowths subsequently acquire a Cavity, or lumen, 

 around which the cells become arranged in a layer. 



The epithelia which cover the skin and line the digestive tube and 

 urogenital organs are thick, as compared with those which line the body- 

 cavity, the vessels, and the synovial cavities. For these thin layers His 

 (1865) introduced the term endothelium. He wrote as follows: 



We are accustomed to designate the layers of cells which cover the serous and 

 vascular cavities as epithelia. Jut all the layers of cells which line the cavities within 



