[Illustration: Fig. 5. A section of the femur, (thigh-bone.) 1, 1, The extremities, showing a thin plate of compact texture, which covers small cells, that diminish in size, but increase in number, as they approach the articulation. 2, 2, The walls of the shaft, which are very firm and solid. 3, The cavity that contains the marrow.] 39. The MUSCULAR TISSUE is composed of many fibres, that unite to form fasciculi, each of which is enclosed in a delicate layer of cellular tissue. Bundles of these fasciculi constitute a muscle. _Observation._ A piece of boiled beef will clearly illustrate the arrangement of muscular fibre. -=-=-=-=-=-=-=-=-=-=-=-= 38. What is said of the osseous tissue? How is the bony matter arranged in different parts of the animal frame? What is said of the use of marrow? 39. Of what is the muscular tissue composed? How may the arrangement of muscular fibre be illustrated? -=-=-=-=-=-=-=-=-=-=-=-= 40. The MUCOUS TISSUE differs from the serous by its lining all the cavities which communicate with the air. The nostrils, the mouth, and the stomach afford examples. The external surface of this membrane, or that which is exposed 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. -=-=-=-=-=-=-=-=-=-=-=-= 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. -=-=-=-=-=-=-=-=-=-=-=-= 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.