I02 THE SURGICAL ANATOMY OF THE HORSE 



from the inferior extremity of the ridge, however, varies directly as 

 the age and size of the animal, and as a rule the larger and older 

 the animal, the further is the septum from the extremity of the 

 ridge. 



The position of the septum then, indicates the lower limit of the 

 opening to be made. The centre of the trephine should also he placed 

 at a spot which is situate one inch in front of the ridge. In 

 young animals, however, it is advisable to select a spot still further 

 forwards, namely, at a distance of an inch and a half to two inches 

 from the ridge, in order that the walls of the molar alveoli may not 

 be injured. 



Upon examination of the area, the ridge of bone is easily felt and 

 seen, and its inferior extremity readily located, since it terminates 

 quite abruptly. In front of the ridge an elongated elevation is observed. 

 This is the elevation which corresponds to the position of the levator 

 labii superioris proprius muscle. This muscle is covered by the thin 

 aponeurotic sheet of the levator labii superioris ahrque nasi muscle, 

 but the latter is here so thin that it does not materially affect 

 the external conformation of the part. 



The hair should now be removed from the surrounding area, and 

 the seat thoroughly cleansed. 



Several different kinds of cutaneous incisions are in use. Some 

 operators make two crossed incisions. Others a T-shaped one, 

 whilst others adopt two incisions meeting one another after the form 

 of the letter V, the apex of which is directed downwards. This 

 last is probably the best kind to be recommended, since the flap of 

 skin is afterwards useful in obscuring from view for a few days the 

 opening into the sinus, and at the same time admits of the free 

 passage outwards of material from within the sinus. 



Such an incision is carefully made, and the skin reflected upwards. 

 There is usually no danger here of injuring either the angular artery 

 or its accompanying vein, since these run obliquely upwards and 



