DISSECTION OF THE ANTERIOR LIMB. 7 



wards. Before fusion, the branch from the 7th gives off a nerve whiCli 

 is distributed to both the levator and the serratus. 



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

 fibres from the dorsal roots of the plexus and from the 8th cervical, 

 but principally 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 perforating branches from the 2ud 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 

 plesus, 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 root 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 separation 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 first 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 Magnus and Levator Anguli Scapula;. — 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-dfgitate 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 outwards and 



