192 THE SURGICAL ANATOMY OF THE HORSE 



sacral. It is an unpaired vessel, and is usually a branch of the right 

 lateral sacral artery. It may, however, come off from the left lateral 

 sacral or the corresponding lateral coccygeal artery. It runs along the 

 inferior aspect of the coccygeal vertebra? between the two compressor 

 muscles to the end of the tail, distributing right and left collateral 

 branches. 



The coccygeal arteries are of surgical importance, since they are the 

 vessels which are concerned in the operation of ' docking,' or amputation 

 of the tail, an operation which is frequently performed, and which is 

 described in Part IV. 



II. — The middle trunk is the Gluteal Artery. This is much the 

 largest of the divisions of the internal iliac artery so far as its 

 diameter is concerned, but on the other hand it is a short vessel. It 

 passes for a short distance on the inner surface of the ilium, and leaves the 

 pelvic cavity by passing out through the greater sacro-sciatic foramen in 

 front of the great sciatic nerve and in company with the anterior gluteal 

 nerves. This foramen, it will be recalled, is covered by the middle 

 gluteus muscle and in close proximity is the deep gluteus. To these 

 muscles the terminal divisions of this vessel are distributed. 



II. — The external trunk splits up into three vessels : 



{a) The Ilio-lumbar or Ilio-Muscular Artery. — This vessel takes a 

 course outwards behind the sacro-iliac joint, running between the iliacus 

 muscle and the bone. On the iliac surface of the ilium a well-marked 

 groove will be found which indicates the course of this vessel. A little 

 below the angle of the haunch the vessel terminates by dividing into 

 a number of branches, which curve upwards across the outer border of 

 the ilium and which are distributed to the middle gluteus, the anterior 

 arm of the superficial gluteus, or to the tensor vagina? femoris muscle. 



[h) The Illo-femoral Aitery. — This vessel is of considerable size only 

 in solipeds, in which it is a larger vessel than the one just described. It 

 passes obliquely downwards between the shaft of the ilium and the iliacus 

 muscle, where there is a distinct groove on the bone for the accommoda- 



