114 THE SURGICAL ANATOMY OF THE HORSE 



On either side the median line the superior surface of the atlas 

 presents three foramina ; these are the antero-internal, antero-external 

 and posterior. The first-named is nearest the median line, from which 

 it is distant 1.25 inch in the average-sized animal, and its opening is 

 placed one inch behind the anterior border of the atlas. It gives 

 exit to the first spinal nerve, whilst through it the cerebro-spinal 

 division of the occipital artery enters the spinal canal. 



From the antero-internal foramen a furrow .75 of an inch long 

 and .25 of an inch deep runs transversely outwards to the antero- 

 external foramen. In this furrow the occipital artery splits up into 

 its terminal branches, namely, the cerebro-spinal, which has already 

 been referred to, and the occipito-muscular. The latter vessel again 

 divides into a number of small branches for the supply of the 

 muscles in the region of the poll. In the furrow, the first spinal 

 nerve splits into its superior and inferior primary divisions. 



The antero-external foramen pierces the atlantal wing, and it 

 gives passage to the occipital artery, enabling this vessel to gain 

 the before-mentioned furrow, and to the inferior primary division 

 of the first spinal nerve. 



The posterior foramen pierces the wing of the atlas obliquely, 

 and it is placed two inches behind the antero-internal foramen, and 

 .25 of an inch from the middle line. This foramen gives passage 

 to the retrograde branch of the occipital artery which anastomoses 

 with the vertebral. 



From what has been said it will be gathered that when the 

 seat of operation is near the front of the atlas, incisions may be 

 made, with little danger of injuring the important structures referred 

 to, within an area 2.5 inches in breadth, which is the distance 

 between one antero-internal foramen and the other. The only structures 

 within this area to be avoided are the small terminal branches 

 of the occipito-muscular artery and the filaments of the superior 

 primary division of the first spinal nerve. In the region of the 



