300 DISEASES OF THE HORSE. 
the theme as a practical collation of facts to be remembered, analyzed, 
applied, and utilized. 
It was the great Bacon who wrote: “The human body may be 
compared, from its complex and delicate organization, to a musical 
instrument of the most perfect construction, but exceedingly liable 
to derangement.” In its degree the remark is.equally applicable to 
the equine body, and if we would keep it in tune and profit by its 
harmonious action we must at least acquaint ourselves with the rela- 
tions of its parts and the mode of their cooperation. 
ANATOMY. 
The bones, then, are the hard organs which in their connection and 
totality constitute the skeleton of an animal (see Plate XXIII). 
They are of various forms, three of which—the long, the flat, and the 
small—are recognized in the extremities. These are more or less 
regular in their form, but present upon their surfaces a variety of 
aspects, exhibiting in turn, according to the requirement of each 
case, a roughened or smooth surface, variously marked with grooves, 
crests, eminences, and depressions, for the necessary muscular attach- 
ments, and, as before mentioned, are connected by articulations and 
joints, of which some are immovable and others movable. 
The substance of the bone is composed of a mass of combined 
earthy and animal matter surrounded by a fine, fibrous enveloping 
membrane (the periosteum) which is intimately adherent to the 
external surface of the bone, and is, in fact, the secreting membrane 
of the bony structure. The bony tissue proper is of two consisten- 
cies, the external portion being hard and “compact,” and called by 
‘the latter term, while the internal, known as the “ spongy ” or “ areo- 
lar tissue,” corresponds to the descriptive terms. Those of the bones 
that possess this latter consistency contain also, in their spongy por- 
tion, the medullary substance known as marrow, which is deposited 
in large quantities in the interior of the long bones, and especially 
where a central cavity exists, called, for that reason, the medullary 
cavity. The nourishment of the bones is effected by means of what 
is known as the nutrient foramen, an opening established for the 
passage of the blood vessels which convey the nourishment necessary 
to the interior of the organ. Concerning the nourishment of the 
skeleton, there are other minutiz, such as the venous arrangement 
and the classification of their arterial vessels into several orders, 
which, though of interest as an abstract study, are not of sufficient 
practical value to refer to here. 
The active organs of locomotion, the muscles (see Plate XXIII), 
speaking generally, form the fleshy covering of the external part of 
the skeleton and surround the bones of the extreniities. They vary 
