HORSE— CARE AND MANAGEMENT. 



47 



for the lodgment of the belly of a mus- 

 cle, as in the case of the scapula. These 

 ■differently shaped bones may, therefore, 

 be classed under the following three 

 heads : 



i st. The long bones consist of the hu- 

 merus, radius, ulna, femur, tibia, and fib- 

 ula ; the metacarpal and metatarsal bones 

 (called, in horsemen's language, the can- 

 non bones), the phalanges (pastern bones), 

 and the ribs. These bones are all divisi- 

 ble into a central cylindrical shaft, and 

 two heads or extremities. The shaft is 

 usually of a prismoid form, dense in tex- 

 ture, and presenting a longitudinal tube in 

 the interior, called the medullary canal, 

 which contains marrow. The heads are 

 broad, to articulate with the next ad- 

 joining bones, and are covered with a thin 

 layer of cartilage, which will be described 

 in the chapter treating of the joints. 

 Their outer surface is a hard, osseous 

 layer, within which is a mass ot cells con- 

 taining red medullary matter, to be pres- 

 ently described. 



2d. The flat bones are composed of 

 two layers of dense tissue, one on each 

 surface, having between them another of 

 a cellular nature, called the diploe. As a 

 matter of course, from their shape, they 

 have surfaces, borders and angles ; in ad- 

 dition to which they have projections, 

 called processes, of various shapes. They 

 consist of the chief bones of the head, 

 the scapula and pelvis. 



3d. The irregular bones comprise the 

 lesser bones of the head and face, the ver- 

 tebras, sacrum, sternum, carpal and tarsal 

 bones, the sesamoid bones, the bones of 

 the foot, and the patellae. They resem- 

 ble the flat bones of 'their structure. 



When microscopically examined, bone 

 is seen to be made up of a dense and 

 homogeneous substance (basis substance), 

 in which are numberless minute cells (cor- 

 puscles of Purkinje). The basis substance 

 is partially fibrous and slightly lamellated, 

 the layers being concentric in long bones 

 and parallel in flat ; it is traversed in all 

 directions (more especially in the long 

 axis, where there is one) by canals (Haver- 

 sian canals), which frequently branch and 

 inosculate, giving passage to vessels and 

 nerves. In certain situations the lamellae 

 separate, and leave between them spaces 

 of various sizes, called cancelli. Besides 

 entering into the composition of the ba- 



sis substance, the lamellae are collected 

 concentrically round the Haversian canals 

 the boundaries of which they form, gener- 

 ally to the extent of ten to fifteen layers. 

 Both the compact and spongy tissues are, 

 therefore, composed of the same element- 

 ary structure, the former being especially 

 intended to afford resistance to violence 

 with as little weight as is consistent with 

 its office, for which reason it is hollowed 

 into a tube ; while the latter is enlarged 

 as much as possible without unnecessarily 

 adding to its weight, the problem being 

 solved by its development in a circular 

 form. 



The Periosteum is a dense, fibrous mem- 

 brane which covers every part of the 

 surface of the bones, excepting their ex- 

 tremities when they enter into the compo- 

 sition of a joint, its place being then 

 occupied by cartilage. (See Joints.) 

 When this membrane covers the bones of 

 the skull it is called pericranium, and 

 when it invests the cartilages of the ribs 

 it receives the name perichondrium. It 

 is full of blood-vessels, especially in the 

 young, and they freely communicate with 

 those of the surrounding soft parts. 

 Hence it is extremely liable to inflamma- 

 tion, either caused by injury to itself or 

 to the parts which cover it. 



The marrow, or medullary substance, is 

 contained in the cavities formed within 

 the bones, being of a yellow color and 

 oily nature in the shafts of the long 

 bones; and more or less red, from the 

 admixture with blood, in the flat and ir- 

 regular bones, and in the heads of the 

 long bones. It is contained within the 

 areolar meshes of a membrane, which 

 lines these cavities, answering to the peri- 

 osteum, which has been already described. 

 This medullary membrane is of excessive 

 tenuity, and is composed of blood-vessels 

 ramifying in fine cellular tissues. The 

 use of marrow in the animal economy is 

 not very clearly demonstrated. 



In the embryo, all the bones originally 

 exist in the state of cartilage, being soft 

 and flexible. By degrees vascular canals 

 are developed within its substance, by the 

 union of its cells in rows. These concen- 

 trate towards some one or more points, 

 which in a long bone are one in the cen- 

 tre of the shaft and one at each extremity. 

 Starting from this point {punetum ossifica- 

 tionis), fibres run out, embracing clusters 



