NECROSIS. 169 



action of the atmosphere and the character of the discharge. 

 The limits of the dead portion are not always easily defined, and 

 at all times its outlines are very irregular. If the skin should 

 be uncut, or be healed, over the seat of the injury, an abscess 

 soon forms, which, if left alone, discharges itself; and if the 

 ulcer is examined by a probe, the bone will be found bare, and 

 perhaps loose. The abscess does not heal until the loose por- 

 tion, called an exfoliation, is removed ; in fact, there is now a 

 fistulous communication between the dead bone and the at- 

 mosphere. The separation of the dead bone is effected by 

 the absorption of the layer of living bone immediately con- 

 tiguous to it, by a process analogous to sloughing of the soft 

 parts. 



When the dead bone is removed — naturally or by surgical in- 

 terference — the surface beneath it gTanulates, and bony matter 

 is formed in the gap, which, however, is not always sufi&cient to 

 fill it, but to round off its edges, and the part is left permanently 

 deficient. 



Free vent must be given to the pus, which is curdy in con- 

 sistence, and foetid in odour — the dead bone examined, and when 

 found to be loose removed by the forceps. If the necrosed 

 portion be a mere thin pellicle, the dilute hydrochloric acid 

 will often dissolve it, and thus save a prolonged process of ex- 

 foliation. 



This form of necrosis often attacks the inferior maxillary bone 

 from the pressure of the curb, and the os pedis, from pressure 

 of a foreign body, as the shoe nail. 



Necrosis totalis affects the shafts of the long bones, more 

 especially the metacarpals and the metatarsals, and also the 

 scapulae. One specimen of the latter is now in the Veterinary 

 College Museum. — (See Photo-lithograph, Plate I., Fig. 6.) 

 The cause of this form is violent concussion, as from galloping 

 or jumping, producing violent ostitis, which from its rapidity 

 does not permit the dense texture of the bone to open out, so 

 as to allow the Haversian canals to accommodate the congested 

 vessels, or permit them to relieve themselves sufficiently by 

 effusion. In addition to tliis, the small amount of exudation 

 which ensues blocks up the canaliculi and lacunae of the bone, 

 arresting its nutrition, and finally destroying its vitality. 



When the vitality of the bone is destroyed, a large deposit 



