384 SPLANCHNOLOGY 
consist of divisions of a larger order which are known as lobes (Lobi); these may be 
separated by layers of connective tissue (Septa interlobares) or by fissures (In- 
cisur 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 mainly 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 (Plicxe 
serosie) or serous ligaments (Ligamenta serosa), but many special terms are in use 
and will be referred to later. 
1The serous membranes form closed saes, except in the female, in which case the uterine 
tubes open into the peritoneal cavity and also communicate indirectly with the exterior. 
