DISSECTION OP THE ANTERIOR LIMB. 7 



The Subcutaneous Thoracic Nerve (Plates 1 and 3) derives its fibres 

 from the dorsal roots of the plexus and from the 8th cervical, but princi- 

 pally from the former. It accompanies the spur vein to near the flank, 

 being distributed with perforating intercostal branches on the deep face 

 of the panniculus carnosus. A branch from it unites with perfoi-ating 

 branches from the 2nd and 3rd intercostal nerves, and turns round behind 

 the limb, to be distributed to the panniculus over the shoulder and arm. 

 The Nerves to the Pectoral Muscles have already been referred to. 

 The nerve to the anterior deep pectoral leaves the fore part of the 

 plexus, deriving its fibres from the 7th and 8th cervical nerves. The 

 nerve to the superficial pectoral muscle (both divisions) derives its fibres 

 from both roots of the median. In general, there are two nerves to the 

 posterior deep pectoral. The first — to the anterior part of the muscle, 

 comes off with the posterior i*oot of the median, the other — to the 

 posterior part of the muscle, comes off in common with the subcutaneous 

 thoracic. 



Directions. — The remaining nerves of the brachial plexus can be more 

 satisfactorily followed after sepai-ation of the limb from the trunk, and 

 the dissector should therefore now proceed as follows : — Pass a cord 

 round the nerves of the plexus as they emerge from between the two 

 divisions of the scalenus, and then cut the roots of the plexus as near 

 their points of origin as possible. Cut also the axillary artery and vein 

 at the fiz-st rib. This will allow the limb to be carried well out from 

 the trunk, so as to expose the serratus magnus and levator anguli 

 scapulae, which are now to be cleaned. 



Serratus Magmis and Levator Anguli Scajmlce. — These muscles are, 

 in the horse, not very distinctly marked off from each other, and have 

 therefore been frequently described as one muscle under the first 

 name. 



The Serratus Magnus (Plate 4) arises from the outer surfaces of the 

 eight (or nine) anterior ribs, its eight slips of origin forming a curved, 

 serrated line which gives to the muscle its name. The posterior four 

 of these slips inter-digitate with slips of origin of the external oblique 

 muscle of the abdomen (Plate 39), and are overspread by the abdominal 

 tunic. It is inserted into a triangular area on the ventral surface of the 

 scapula near its dorsal angle, and, in common with the next muscle, into 

 another triangular area at the cervical angle. 



Action. — It pulls the dorsal angle of the scapula downwards and 

 backwards on the chest- wall, causing the shoulder-joint at the same 

 time to move upwards and forwards ; but when the limbs are fixed, it 

 can become a muscle of inspiration, pulling the ribs upwai-ds and for- 

 wards. In the standing posture of the animal at I'est, the chest is, in 

 a manner, slung on the fore limbs by means of the right and left ser- 

 ratus muscles. 



