SPLANCHNOLOGY 
This branch deals with the viscera of the digestive, respiratory, and uro- 
genital systems.' Each of these systems consists fundamentally of a tube or tract, 
which is lined with mucous membrane (Tunica mucosa) and communicates with 
the exterior at one end or both. Thus the epithelium of the mucous membranes 
is continuous with the epidermis at the various natural openings. In addition to 
this fundamental part there are to be considered aggregations of secreting cells 
known as glands, muscular tissue, fibrous membranes, serous membranes, vessels 
and nerves. 
Mucous Membranes.—These vary much in thickness, color, and other char- 
acters. In many places they form folds (Plicee mucosz) which may be temporary 
or permanent. In other places they form ridges (Rugze). With certain excep- 
tions they are moistened by a viscid secretion, termed mucus, which is derived 
from glands or goblet cells of the epithelium. The membrane consists of two dis- 
tinct parts: the epithelium, which forms the free surface and is protective and secre- 
tory; and the lamina propria, a layer of connective tissue which contains and sup- 
ports the peripheral ramifications of the vessels and nerves. The mucous mem- 
brane is connected with surrounding structures by areolar submucous tissue 
(Tela submucosa). In many places there is a layer of unstriped muscle, the mus- 
cularis mucose, in the deepest part of the mucous membrane. In many situa- 
tions the tunica propria presents numerous elevations, known as papilla. When 
small (microscopic), the papillae do not modify the surface of the mucous membrane, 
since the epithelium levels up the depressions between them, but when large (ma- 
croscopic), they are conspicuous surface features, and are named according to their 
shape as conical, foliate, ete. 
Glands.—The term gland (Glandula) is usually understood to mean an aggre- 
gation of epithelial cells, the secretion of which is extruded on the free surface of the 
membrane, or is conveyed away in the blood or lymph stream.” Glands are divided 
according to their form into two chief classes, tubular and alveolar, each of which 
may be simple or compound; many, however, combine the characters of both types 
and are termed tubulo-alveolar or alveolo-tubular. A simple tubular gland is a cylin- 
drical depression lined by epithelium which is continuous with that of the surround- 
ing mucous membrane, from which it developed originally as an outgrowth. The 
deeper part of such a gland is termed the fundus, and here the epithelium is differ- 
entiated and has taken on secretory function. The more superficial part which 
conveys the secretion to the surface is called the duct; in it the epithelium resembles 
more or less closely that of the surrounding surface. Many glands are microscopic, 
while others are large organs. The larger ones are composed of subdivisions known 
as lobules (Lobuli), which are held together by areolar interlobular tissue; each 
lobule has its duct and by the union of these ducts there is formed an excretory 
duct (Ductus excretorius), through which the secretion is conveyed. Some glands 
_ 1 The term viscus is applied in general to the organs which are contained in the body cavities. 
Tt is usual to exclude the heart, except when considering the thoracic organs topographically. 
Certain other organs which do not belong to these systems are usually considered with them as a 
matter of convenience. 
| * But unicellular glands are recognized in lower forms, and in higher forms the goblet-cells 
" many mucous membranes have the same function. Other organs which are not epithelial in 
structure are commonly classed as glands; such, for example, are the lymph glands and nodules, 
the thymus, and the spleen. They are usually termed ductless or vascular glands, and are des- 
ribed with the organs with which they are associated anatomically. 
