288 THE ANATOMY OF THE HORSE. 



as fiir as is necessary to bring the origin of the external ol)liq\ie muscle 

 of the abdomen into view. Notice on the inner surfiice of the muscle 

 ramifying nerves, and anteriorly the spur vein accompanied by a small 

 branch of the external thoracic artery. 



Perforating Nerves. The nerves seen descending on the inner 

 surface of the panniculus are perforating branches derived from the 

 intercostal trunks, and from the last dorsal and first lumbar nerves. 

 These perforating nerves appear along a curved line a few inches below 

 the origin of the external oblique. They supply the panniculus, and give 

 cutaneous twigs through it to the overlying skin. 



A perforating branch from the 2nd lumbar nerve appears close to the 

 bony prominence of the haunch, and descends to the skin on the h'oni, 

 of the thigh, A perforating branch from the 3rd lumbar nerve appears 

 below the same bony prominence, and two inches below the point of exit 

 of the preceding nerve. It is accompanied by a branch of the circum- 

 flex iliac artery, with which it descends to the thigh, internal to the last 

 described branch. 



The Subcutaneous Thoracic Nerve (Plate 1). This will be found 

 ninning horizontally backwards on the inner surface of the panniculus, 

 behind the shoulder, and in company with the vessels of the same name. 

 It comes from the brachial plexus. 



Perforating Vessels. Small un-named branches, mostly branches 

 of the intercostal vessels, appear at the same points as the nerves. 



The Abdominal Tunic (Plate 38). This is a great expansion of 

 yellow elastic tissue which is spread over the inferior and lateral walls 

 of the abdomen. It is nearly co-extensive with the external oblique 

 muscle, to which it is adherent. It is thickest in its posterior part, 

 near the linea alba ; and becomes gradually tliinner as it is tx'aced out- 

 wards over the muscular part of the external oblique, and forwards 

 beneath the posterior deep pectoral. Posteriorly it furnishes the sus- 

 pensory ligaments of the prepuce, or analogous slips to the mammaiy 

 gland. The tunic acts as an admirable clastic abdominal bandage, 

 assisting the muscles to support the heavy abdominal viscera, and 

 adapting the wall of the abdomen to the varying volume of its contents. 



Directions. — The abdominal tunic must be entirely removed. This is 

 an operation requiring time and care, for the tunic is intimately adherent 

 to the tendon of the external oblique muscle, especially in its anterior 

 half. Transverse incisions should be made through it, taking care not 

 to cut the fibres of the subjacent tendon, which will be recognised by its 

 different colour and texture. Then seize the cut edges of the tunic with 

 the forceps, and tear it off in strips forwards and backwards. Proceed 

 in this way until the whole of it has been torn away. 



Muscles of the Abdominal Wall. On each side there are four of these, 

 viz., the obliquus abdominis extenius, the obliquus abdominis intemus, 



