328 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



E. THE NERVOUS SYSTEM AND SENSE ORGANS OF THE PIGEON 



i. The spinal nerves and the sympathetic system. — Carefully remove the 

 remaining viscera from one-half of the trunk. Note the ventral rami of the 

 spinal nerves passing laterally along the dorsal body wall between the ribs in 

 the trunk region. Trace them toward the vertebral column and note, at the 

 points where they emerge from the vertebrae, the ganglia of the sympathetic 

 system lying on the spinal nerves and the delicate white cords of the sympathetic 

 system extending between the ganglia along the sides of the vertebral column. 



a) Spinal nerves and limb plexi: In the neck the cervical spinal nerves will 

 be seen by separating the vertebral column from the skin. They pass out at 

 segmental intervals. The vagus nerve is the white cord which passes ventral 

 to the proximal portions of the cervical nerves. 



The ventral rami of the last cervical nerves together with that of the first 

 of the trunk form the brachial plexus to the wing. This is a network, formed 

 by the union of branches of four stout nerves, which receives a small branch 

 from the succeeding nerve. 



The next five ventral rami pass out between the ribs. Following them is 

 the lumbosacral plexus, divisible into three parts, the lumbar, the sacral, and the 

 pudendal plexus. The lumbar plexus is formed by three nerves; from it nerves 

 pass into the thigh. The sacral plexus arises from the union of five nerves, the 

 first of which is the same as the third nerve contributing to the lumbar plexus. 

 These five unite to produce a large trunk, the sciatic nerve, which passes along 

 the dorsal side of the thigh between the muscles, and proceeds down the leg. It 

 will be found by separating the muscles along the middle of the dorsal surface 

 of the thigh. It courses alongside the femoral artery and vein. 



The remaining spinal nerves posterior to. the sacral plexus form the pudendal 

 plexus and pass obliquely posteriorly to the tail and cloacal region. 



b) The sympathetic system: This has already been identified on the sides of 

 the vertebral column. It consists on each side of a chain of two cords and 

 segmental ganglia. One of the cords passes ventral to the head of the rib, the 

 other dorsal to it. A sympathetic ganglion lies fused to each spinal nerve in 

 the trunk region as the latter emerges from the vertebral column. On scraping 

 off one of these sympathetic ganglia, the spinal ganglion belonging to the spinal 

 nerve will be found dorsal to it. At about the middle of the rib-bearing region 

 a plexus of nerves and ganglia will be seen extending ventrally from the main 

 sympathetic cords and surrounding the dorsal aorta and its main branches to 

 the digestive tract. This is the coeliac plexus. Posterior to this region the 

 sympathetic cords are reduced and consist of a single trunk on each side. A 

 sympathetic cord accompanies the pudendal plexus and has a ganglion in the 

 middle of this plexus. Anteriorly the sympathetic cords pass across the ventral 

 side of the brachial plexus, having ganglionic enlargements on the latter, and 

 then enter the vertebrarterial canals. 



