276 DISEASES OF THE HORSE. 



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 

 relations 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. They are of various 

 forms, three of which — ^the long, the flat, and the small — are recog- 

 nized 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 depres- 

 sions, for the necessary muscular attachments, and, as before men- 

 tioned, are connected by articulations and joints, of which some are 

 immovable and others of a movable kind. 



The substance of the bones 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 ex- 

 ternal surface of the bone, and is, in fact, the secreting membrane of 

 the bony structure. The bony tissue proper is of two consistencies, 

 the external portion being hard and " compact," and called by the 

 latter term, while the internal, known as the " spongy," or " areolar 

 tissue," corresponds with the descriptive terms. Those of the bones 

 which 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 necessary nourishment to the interior 

 of the organ. There are other minutiae concerning the nourishment of 

 the skeleton, 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, speaking generally, 

 form the fleshy covering of the external part of the skeleton and 

 surround the bones of the extremities. They vary greatly in shape 

 and size, being flat, triangular, long, short, or broad, and are 

 variously and capriciously named, some from their shape, some from 

 their situation, and others from their use, and thus we have abductors 

 and adductors, the pyramidal and orbicular, the digastricus, the 

 vastus, and so on. Those which are under the control of the will. 



