384 SPLANCHNOLOGY 



consist of divisions of a larger order which are known as lobes (Lobi) ; these may be 

 separated bj^ laj'ers of connective tissue (Septa interlobares) or by fissures (In- 

 cisurse interlobares) . 



There is no correspondence between the size of a gland and the number of its excretory ducts. 

 Thus the largest gland in the body, the liver, has a single excretory duct, while some small glands 

 have many. 



Muscular Tissue. — Most of the hollow organs are provided with a muscular 

 coat (Tunica muscularis) outside of the mucous membrane. This is in great part 

 composed of strata of unstriped muscle, but in certain places — and especially in the 

 vicinity of the natural apertures — it consists of striped muscle. Some of the solid 

 organs contain muscular tissue in their capsule or stroma^. 



Fibrous Membranes. — Many viscera are enclosed by a fibrous coat (Tunica 

 fibrosa) . In the case of glands such an enveloping membrane is usually called the 

 capsule. Other membranes of a like character are known as a tunica albuginea 

 or tunica adventitia; the former consist mainlj^ of dense white fibrous tissue, while 

 the latter usually contain many elastic fibers and are looser in texture. 



Serous Membranes. — These are thin membranes which line the body-cavities 

 and cover more or less the external surface of the viscera contained therein. "^ They 

 include the peritoneum in the abdomen, and the pleura and the deep layer of the 

 pericardium in the thorax. Their free surface is formed by a mesothelium of 

 flat cells; it is smooth and glistening and is moistened by a film of serum, thus re- 

 ducing friction to a minimum. The external surface is in most places connected 

 with the structure covered by areolar subserous tissue (Tela subserosa) which 

 often contains fat. The part which lines the wall of a cavity is termed the parietal 

 layer (Lamina parietalis), while the visceral layer (Lamina visceralis) is that which 

 forms the serous coat (Tunica serosa) of the viscera. • Double layers which connect 

 viscera with the wall or with each other are in general called serous folds (Plicae 

 serosse) or serous ligaments (Ligamenta serosa), but many special terms are in use 

 and will be referred to later. 



1 The sei'ous membranes form closed sacs, except in the female, in which case the uterine 

 tubes open into the peritoneal cavity and also communicate indirectly with the exterior. 



