GENERAL ANATOMY OF BONE. 35 



superincumbent pressure is well exhibited in various parts of the skeleton, such 

 as the human foot, and more especially in the vaulted roof of the cranium. 



Bones are divisible into four classes : Long, Short, Flat, and Irregular. 



The Long Bones are found chiefly in the limbs, where they form a system of 

 levers, which have to sustain the weight of the trunk, and to confer extensive 

 powers of locomotion. A long bone consists of a lengthened cylinder or shaft, 

 and two extremities. The shaft is a hollow cylinder, the walls consisting of dense 

 compact tissue of great thickness in the middle, and becoming thinner towards the 

 extremities ; the spongy tissue is scanty, and the bone is hollowed out in its interior 

 to form the medullary canal. The extremities are generally somewhat expanded 

 for greater convenience of mutual connection, for the purposes of articulation, and 

 to afford a broad surface for muscular attachment. Here the bone is made up of 

 spongy tissue with only a thin coating of compact substance. The long bones are, 

 the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpal and metatarsal 

 bones, and the phalanges. 



Short Bones. Where a part is intended for strength and compactness, and the 

 motion at the same time slight and limited, it is divided into a number of small 

 pieces united together by ligaments, and the separate bones are short and com- 

 pressed, such as the bones of the carpus and tarsus. These bones, in their struc- 

 ture, are spongy throughout, excepting at their surface, where there is a thin crust 

 of compact substance. 



Flat Bones. Where the principal requirement is either extensive protection, or 

 the provision of broad surfaces for muscular attachment, we find the osseous struc- 

 ture remarkable for its slight thickness, becoming expanded into broad flat plates, 

 as is seen in the bones of the skull and shoulder-blade. These bones are composed 

 of two thin layers of compact tissue, inclosing between them a variable quantity 

 of cancellous tissue. In the cranial bones, these layers of compact tissue are 

 familiarly known as the tables of the skull ; the outer one is thick and tough, the 

 inner one thinner, denser, and more brittle, and hence termed the vitreous table. 

 The intervening cancellous tissue is called the diplo'e. The flat bones are, the 

 occipital, parietal, frontal, nasal, lachrymal, vomer, scapulae, ossa innominata, sternum, 

 and ribs. 



The Irregular or Mixed bones are such as, from their peculiar form, cannot be 

 grouped under either of the preceding heads. Their structure is similar to that of 

 other bones, consisting of a layer of compact tissue externally, and of spongy 

 cancellous tissue within. The irregular bones are. the vertebrse, sacrum, coccyx, 

 temporal, sphenoid, ethmoid, superior maxillary, inferior maxillary, palate, inferior 

 turbinated, and hyoid. 



Surfaces of Bones. If the surface of any bone is examined, certain eminences 

 and depressions are seen, to which descriptive anatomists have given the following 

 names. 



A prominent process projecting from the surface of a bone, which it has never 

 been separate from, or movable upon, is termed an apophysis (from arto<pv<n, an 

 , excrescence] ; but if such process is developed as a separate piece from the rest of 

 the bone to which it is afterwards joined, it is termed an epiphysis (from enifvais, 

 an accretion). 



These eminences and depressions are of two kinds: articular and non-articular. 

 Well-marked examples of articular eminences are found in the heads of the 

 humerus and femur; and of articular depressions, in the glenoid cavity of 

 the scapula, and the acetabulum. Non-articular eminences are designated 

 according to their form. Thus, a broad, rough, uneven elevation is called a 

 tuberosity ; a small, rough prominence, a tubercle; a sharp, slender, pointed 

 eminence, a spine; a narrow, rough elevation, running some way along the surface, 

 a ridge, or line. 



The non-articular depressions are also of very variable form, and are described 

 as fossre, grooves, furrows, fissures, notches, etc. These non-articular eminences 

 and depressions serve to increase the extent of surface for the attachment of liga 



