306 THE HORSE. 
retains its shape and rigidity, but becomes much whiter in colour, and 
is rendered extremely brittle. In fact, the mineral salts entering into its 
composition are left, but the animal matter binding them together is com- 
pletely decomposed and carried off in a gaseous form. On the other hand, 
by immersing a bone for two or three weeks in diluted hydrochloric acid, 
the earthy salts are dissolved, while the animal matter is untouched. 
Here the bone retains its original shape, but it is soft and flexible ; and 
instead of presenting its usual opaque yellowish-white colour, it 1s semi- 
transparent, and resembles the ordinary gelatine of the shops. According 
to Berzilius, bone is chemically composed of the following constituents— 
namely, cartilage, reducible to gelatine by boiling; blood-vessels ; phos- 
phate of lime ; carbonate of lime ; fluate of lime ; phosphate of magnesia ; 
soda and chloride of sodium. 
Considered mechanically, the bones form the framework of the animal 
machine. In the limbs they are hollow cylinders, admirably fitted by 
their shape and texture to resist violence and support weight. In the 
trunk and head they are flattened and arched, to protect the contents of 
the cavities they form, and to provide an extensive surface for the attach- 
ment of muscles. In certain situations their exterior is raised into pro- 
jections called processes, which serve as levers for the muscles to act upon ; 
in others they are grooved into smooth surfaces for the easy gliding of 
tendons, when these are stretched between the fleshy part of a muscle and 
one of its attachments. Lastly, they sometimes present a large hollow for 
the lodgment of the belly of a muscle, as in the case of the scapula. These 
differently shaped bones may, therefore, be classed under the following 
three heads :— 
Ist. The long bones consist of the humerus, radius, ulna, femur, 
tibia, and fibula; the metacarpal and metatarsal bones (called, in 
horsemen’s language, the cannon bones), the phalanges (pastern bones), 
and the ribs. These bones are all divisible into a central cylindrical 
shaft, and two heads or extremities. The shaft is usually of a prismoid 
form, dense in texture, and presenting a longitudinal tube in the interior, 
called the medullary canal, which contains marrow. The heads are broad, 
to articulate with the next adjoining 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 of cells containing red medullary matter, to be presently 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 diplée. As a matter of course, from their shape, they have surfaces, 
borders, and angles; in addition 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 vertebrae, sacrum, sternum, carpal and tarsal bones, the sesamoid 
bones, the bones of the foot, and the patelle. They resemble the flat 
bones in 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 (corpuscles 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 (Haversian canals), which 
frequently branch and inosculate, giving passage to vessels and nerves. 
