DESCRIPTIVE AND SURGICAL ANATOMY. 



The Skeleton. 



THE entire skeleton in the adult consists of 200 distinct bones. These are 



The Spine or vertebral column (sacrum and coccyx included, 26 



Cranium 8 



Face 14 



Os hyoides, sternum, and ribs 26 



Upper extremities 6-i 



Lower extremities 62 



200 



In this enumeration, the patellae are included as separate bones, but the 

 smaller sesamoid bones, and the ossicula auditus, are not reckoned, The teeth 

 belong to the tegumentary system. 



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



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

 which have to sustain the weight of the trunk, and to confer the power 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 com- 

 pact 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 sub- 

 stance. The long bones are, the humerus, radius, ulna, femur, tibia, fibula, 

 metacarpal, and metatarsal bones, and the phalanges. The clavicle is also usually 

 reckoned as a long bone. 



Short Bones. Where a part of the skeleton is intended for strength and 

 compactness, and its motion is 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 compressed, such as the bones of the carpus and tarsus. 

 These bones, in their structure, are spongy throughout, excepting at their sur- 

 face, 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 

 structure expanded into broad flat plates, as is seen in the bones of the skull 

 and the shoulder-blade. These bones are composed of two thin layers of com- 

 pact 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 diploe. The flat bones are, the occipital, parietal, 

 frontal, nasal, lachrymal, vomer, scapulae, ossa innominata, sternum, and ribs. 



115 



