44 THE SURGICAL ANATOMY OF THE HORSE 



therefore to be incised. After the reflection of the cutaneous flap, 

 twigs from the pes anserinus which ramify on the outer aspect of 

 the muscle, are to be avoided whilst making the deeper incision. 



The centre of the trephine should be brought one inch and a hah 

 further forwards and half an inch downwards in operating on the 

 fifth lower molar, and in this case also it is necessary to make an 

 incision in the masseter muscle. 



The operation on the fourth molar probably presents much the 

 greatest difficulty. The seat in this case is one inch forwards and a 

 quarter of an inch downwards from that in the previous case. We 

 are here in close proximity to the submaxillary artery and vein, and 

 the accompanying duct of the parotid gland. To keep clear of these 

 the pulsation of the artery is felt, and remembering that the artery 

 is the most anterior of these structures we know that in operating well 

 in front of the artery we are clear of the vein and duct. 



But at a point which is usually about half an inch above our seat, 

 the submaxillary artery gives off its inferior labial branch, which, in this 

 position of the head, runs forwards and to a slight extent obliquely 

 upwards, to pass beneath the depressor labii inferioris muscle. This 

 vessel is easily avoided if our measurement be carefully taken from 

 the inferior border of the lower jaw, from which the distance to the seat 

 of operation is one inch and a quarter. Regarding the first three 

 molars of the lower jaw little difficulty is experienced in this respect 

 since the seat of operation in each case is placed about three-quarters 

 of an inch above the edge of the inferior maxilla, and a distance of one 

 inch and a half separates the centre of the trephined opening from 

 that of the root of the adjacent tooth. 



The foregoing measurements were taken from the skull ot a five- 

 year-old animal, fifteen hands high. From this age onwards, the seat 

 of operation will approach progressively nearer the alveolar edges 

 of the maxillary hones. 



