xposed to the air, is soft, and bears some
resemblance to the downy rind of a peach. It is covered by a viscid
fluid called _mu'cus_. This is secreted by small _gland-cells_,
called _ep-i-the'li-a_, or secretory cells of the mucous membrane.
The use of this membrane and its secreted mucus is to protect the
inner surface of the cavities which it lines.

_Observation._ A remarkable sympathy exists between the remote parts
of the mucous membrane. Thus the condition of the stomach may be
ascertained by an examination of the tongue.

41. The NERVOUS TISSUE consists of soft, pulpy matter, enclosed in
a sheath, called _neu-ri-lem'a_. This tissue consists of two
substances. The one, of a pulpy character and gray color, is called
_cin-e-ri'tious_, (ash-colored.) The other, of a fibrous character and
white, is named _med'ul-la-ry_, (marrow-like.) In every part of the
nervous system both substances are united, with the exception of the
nervous fibres and filaments, which are solely composed of the
medullary matter enclosed in a delicate sheath.

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40. How does the mucous differ from the serous tissue? What is the
appearance of the external surface of this membrane? Where is the
mucus secreted? What is the use of this membrane? 41. Of what does the
nervous tissue consist? Describe the two substances that enter into
the composition of the nervous tissue.

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CHAPTER III

CHEMISTRY OF THE HUMAN BODY.

42. An ULTIMATE ELEMENT is the simplest form of matter with which we
are acquainted; as gold, iron, &c.

43. These elements are divided into _metallic_ and _non-metallic_
substances. The metallic substances are _Po-tas'si-um_, _So'di-um_,
_Cal'ci-um_, _Mag-ne'si-um_, _A-lu'min-um_, _I'ron_, _Man'ga-nese_, and
_Cop'per_. The non-metallic substances are _Ox'y-gen_, _Hy'dro-gen_,
_Car'bon_, _Ni'tro-gen_, _Si-li'-ci-um_, _Phos'phor-us_, _Sul'phur_,
_Chlo'rine_, and a few others.

44. POTASH (potassium united with oxygen) is found in the blood, bile,
perspiration, milk, &c.

45. SODA (sodium combined with oxygen) exists in the muscles, and in
the same fluids in which potash is found.

46. LIME (calcium combined with oxygen) forms the principal ingredient
of the bones. The lime in them is combined with phosphoric and
carbonic acid.

47. MAGNESIA (magnesium combined with oxygen) exists in the bones,
brain, and in some of the animal fluids; as milk.

48. SILEX (silic