73. The PERIOSTEUM is a firm membrane immediately investing the bones, except where they are tipped with cartilage, and the crowns of the teeth, which are protected by enamel. This membrane has minute nerves, and when healthy, possesses but little sensibility. It is the nutrient membrane of the bone, endowing its exterior with vitality; it also gives insertion to the tendons and connecting ligaments of the joints. -=-=-=-=-=-=-=-=-=-=-=-= 72. What is the membrane called that invests the bones? That covers the cartilage? That invests the skull? Explain fig. 6. 73. Describe the periosteum. -=-=-=-=-=-=-=-=-=-=-=-= 74. There are two hundred and eight[3] bones in the human body, beside the teeth. These, for convenience, are divided into four parts: 1st. The bones of the _Head_. 2d. The bones of the _Trunk_. 3d. The bones of the _Upper Extremities_. 4th. The bones of the _Lower Extremities_. [3] Some anatomists reckon more than this number, others less, for the reason that, at different periods of life, the number of pieces of which one bone is formed, varies. _Example._ The breast-bone, in infancy, has _eight_ pieces; in youth, _three_; in old age, but _one_. 75. The bones of the HEAD are divided into those of the _Skull_, _Ear_, and _Face_. 76. The SKULL is composed of eight bones. They are formed of two plates, or tablets of bony matter, united by a porous portion of bone. The external tablet is fibrous and tough; the internal plate is dense and hard, and is called the _vit´re-ous_, or glassy table. These tough, hard plates are adapted to resist the penetration of sharp instruments, while the different degrees of density possessed by the two tablets, and the intervening spongy bone, serve to diminish the vibrations that would occur in falls or blows. 77. The skull is convex externally, and at the base much thicker than at the top or sides. The most important part of the brain is placed here, completely out of the way of injury, unless of a very serious nature. The base of the cranium, or skull, has many projections, depressions, and apertures; the latter affording passages for the nerves and blood-vessels. -=-=-=-=-=-=-=-=-=-=-=-= 74. How many bones in the human body? How are they divided? 75-81. _Give the anatomy of the bones of the head._ 75. How are the bones of the head divided? 76. Describe the bones of the skull. 77. What is the form of the skull? What does the base of the skull present? -=-=-=-=-=-=-=-=-=-=-=-=