74 GLANDS; MUCOUS AND SEROUS MEMBRANES. 



Organs are aggregations of the elementary cells and tissues 

 in varying proportions and varying modes of arrangement. 

 The tissues of many organs can be conveniently divided into 

 parenchymatous tissue and sustentacular tissue. 



The parenchyma of organs consists of the cells or portions 

 that are the active and essential agents in carrying on the 

 functions of the organ, such as the glandular epithelium of 

 glands, the air-vesicles of the lungs, etc. The sustentacular 

 tissue (often called interstitial tissue) of organs consists of 

 their connective-tissue framework, permeating and supporting 

 all their parts. It is usually a fibro-elastic or condensed 

 areolar tissue. Different parts or divisions of the sustentacu- 

 lar tissue can often be distinguished, such as the membranous 

 envelopes or capsules surrounding organs; trabeculie or septa 

 (interlobular tissue) traversing the interior of organs and 

 dividing them into lobes and lobules ; tracts of fibrous tissue 

 (perivascular tissue, etc.) surrounding and supporting the 

 vascular and nerve branches, and ducts, in their ramifications 

 through the organ ; and a delicate all-pervading network of 

 fibrous tissue (intralobular tissue) embracing and sustaining 

 the minutest subdivisions of the visceral and parenchymatous 

 substance. In addition to their sustentacular and parenchy- 

 matous elements organs possess as a matter of course a blood, 

 lymphatic, and nerve supply. 



