BONE, NORMAL ANATOMY. 



431 



them in so complicated a piece of machinery 

 as the skeleton. These varieties have been 

 reduced by anatomists to four classes, viz. 1. 

 the long or cylindrical ; 2. the broad or flat; 

 3. the short or thick ; and 4. the mixed or irre- 

 gular bones. The long bones are distinguished 

 by their length, which greatly exceeds their 

 other dimensions. They are to be found only 

 in the extremities, and are adapted for locomo- 

 tion and for supporting the weight of the body. 

 They are never exactly cylindrical, being al- 

 ways contracted in the middle or shaft, and 

 enlarged at each end ; and their transverse sec- 

 tion is oval or triangular, never round. The 

 broad or flat bones are thin, generally arched, 

 and fitted to protect delicate organs ; we find 

 the best specimens of them in the cranium. 

 The short have nearly equal length, breadth, 

 and thickness; they are seen in the carpus and 

 tarsus. The mired or irregular bones are 

 usually classed with the short, but it is more 

 convenient to separate them : the vertebrae are 

 good examples of these. The ribs and bones 

 of the pelvis may also be ranged with them, 

 combining the characters of two of the pre- 

 ceding classes. Each of these divisions exhibits 

 certain peculiarities of structure to which we 

 shall allude hereafter.* 



If we prepare bones by careful maceration 

 and drying, and then saw them through, or, 

 what is better, fracture them with a smart blow 

 of a hammer, we observe the density of texture 



to differ very much in different portions. The 

 outer part is generally much more dense and 

 close than the interior, and is called the com- 

 pact, vitreous, or cortical substance. The inte- 

 rior, open and areolar in its appearance, is the 

 spongy, cancellated, or reticular substance. 

 These two are arranged in a peculiar manner 

 in each class of bone. In the long there is a 

 considerable thickness of compact substance in 

 the shaft, surrounding a cavity, and but little 

 of the spongy, whilst towards the ends the 

 compact gradually becomes thin as paper, the 

 spongy increasing in quantity and filling up all 

 the interior, as if formed by the expansion of the 

 compact tissue (fig- 186, a). In the flat bones 

 the compact substance is formed into two plates 

 with a thin stratum of spongy substance called 

 the diploe between (fig. 186, b). In very thin 

 bones the diploe is often absent. The short 

 bones are spongy throughout, with a thin layer 

 of the compact tissue on the surface ; they are 

 like the extremities of the long bones: and the 

 irregular, resembling in shape two or more of 

 the former classes, have a corresponding arrange- 

 ment of the two tissues. 



A vertical section of three long bones is re- 

 presented at Jig. 186, where A is the head and 

 neck of the femur, and B the upper extremities 

 of the tibia and fibula : a indicates the com- 

 pact tissue; b the reticular; at c it may be seen 

 how thin is the layer of compact tissue cover- 

 ing the head of the femur. 



Fig. 186. 



In the shaft of the long bones there exists a 

 cavity of considerable size filled with marrow, 

 and called the medullary cavity. This is widest 

 in the centre, gradually getting smaller towards 



* See further particulars respecting foramina, 

 processes, epiphyses, &e. as connected with mecha- 

 nical contrivance, under the article SKULKTON. 



the extremities, where its place is occupied with 

 spongy substance. The interior of this cavity 

 is rough ; bony processes project into it, and 

 form a kind of net-work resembling the spongy 

 substance at the ends, but with more open and 

 less regular cells. By some anatomists the 

 term spongy is confined to the cellular arrange- 

 ment at the ends, that in the middle being 



