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AREOLAR TISSUE, TESBOyS, ASD MUSCLES. 199

The Genito-urinary System is that which, commencing at the kidney.-,
lines the urinary passages of both sexes, passing through the ureters,
bladder, and uretlu-a, also the sexual organs, as the seminal ducts and
vesicles of the male, the vagina, uterus, and Fallopian tubes of the
female.
The Gadro-pneumonic System lines the alimentary canal and all ducts
and glands which o|>en into it ; this system invests the air-jjassages
from the opening of the nostrils to the air-vesicles of the Imigs. It
also lines the frontal sinuses and air-cells which communicate with them
(jr the air-passages. The membrane passes from the nasal chamber
through the Eustachian tube to the ear, also through the lachrymal
ducts to the eyes and lachrymal glands. The latter gland is developed
from a solid ingro^\th of the conjtmctiva.
The construction of the mucous membrane is very similar to that of
the skin, being develojjed from two layers of the blastoderm—the skin
from the epiblast and mesoblast, the mucous membrane (with certain
exceptions) from the hypo- and mesoblast : it is divided into two layers,
epithelium and corium, separated by an intermediate or basement-mem-
brane.
Epithelium of Mucous Membeaxe.—The epithelial layer is
e. it passes over and
the most constant part of mucous membrane— /.
into parts where the corium cannot be traced, as in the alveoli of the
luug-s and upon the cornea of the eye ; it is develojjed, with the excep-
tion hereafter given, from the lower layer (h^-]Xlblastic) of the blasto-
derm. This laver is variotisly classified, either according to its function
or to the shape and arrangement of the cells entering into its formation,
as simple, stratified, and transitional, the simple variety being again
subdivided into pavement, columnar, spheroidal or glandular, and cili-
ated.
"When classified according to function, it is arranged as protective and
secretory, the first division being made up of stratified, transitional, cili-
atetl, and pavement varieties ; the latter is divided into columnar and
spheroidal (glandular).
The Epitheliu:m of the Oral Cavity.—The epithelium within
the oral cavity is reg-ardcd as squamous stratified epithelium ; the same
variety is also found in the lower part of the larynx, upon the edges of
the epiglottis, the true vocal cords, and in the cesophagus ; also in the
anterior two-thirds of the nasal chamber.
The general arrangement of cells in the epithelium is very similar to
that of the epiderm ; its deep or Malpighian layer contains very little
pigment, and is columnar in form, though this is not the case in embiy-
onal life—the stratum granulosum and stratum lucidum of the epiderm
are not present ; but with this exception the development. gro\nh. main-
tenance, and desquamation are the same. It is analogous with the skin,
and is develo]^ from the same layer of the blastoderm, which extends
internally as far as the palato-glossal fold, and sometimes farther.
Should any portion of the mucous membrane of the mouth become
constantly exjiosed to the action of the atmosphere as the result of surgi-
cal operations or other cause, it will assume the horny character of the
skin. In like manner, should any part of the skin become part of the