322 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



a) Spinal nerves and limb plexi: Carefully expose the spinal nerves of the 

 trunk, avoiding injury to the sympathetic system. These nerves, called the dor- 

 sal spinal nerves, run along the sutures between the costal plates of the cara- 

 pace. In most cases each consists of two branches, a smaller dorsal ramus 

 and a larger ventral ramus. On tracing these toward the vertebral column they 

 will be found to come from a large ganglion, the dorsal or spinal ganglion, situated 

 in contact with the center of the centrum. 



Expose the brachial plexus in the depression between the neck and the dorsal 

 end of the scapula. It is generally formed by the cross-unions between the 

 ventral rami of the last four cervical spinal nerves and the first dorsal spinal 

 nerve. The last-named nerve may be identified as the one in front of the first 

 typical rib (really the second rib). The four cervical nerves form a complex 

 network on the surface of the shoulder muscles. From this network the large 

 median nerve proceeds along the anterior surface of these muscles and the 

 smaller ulnar and radial nerves along the posterior surface ; the radial is the most 

 dorsally situated one. The first dorsal nerve sends a branch near its ganglion 

 to the brachial plexus and is then distributed to the carapace just posterior to 

 the fore limb. 



The next six dorsal nerves are similar to the first description given. The 

 ventral rami of the eighth, ninth, and tenth dorsal nerves together with the two * 

 sacral nerves form the lumbosacral plexus for the hind limb. This lies on the 

 medial surface of the muscles covering the ilium and is found by carefully separat- 

 ing the ilium with its muscles from the median region. The branches from the 

 three dorsal nerves unite to a sort of knot from which several nerves proceed to 

 the anterior part of the leg. The two sacral nerves, receiving also a contribu- 

 tion from the tenth dorsal nerve, unite to form a large trunk, the sciatic nerve, 

 situated among the muscles on the posterior side of the leg. 



There is a pair of caudal spinal nerves corresponding to each caudal vertebra, 

 but these need not be looked for. There are nine pairs of cervical nerves which 

 will be found by looking in the neck at the same level as the level of emergence 

 of the nerves of the brachial plexus. 



b) The sympathetic system: Locate the vagus nerve in the neck. It is the 

 conspicuous white cord running along the side of the neck. Trace it posteriorly. 

 The sympathetic trunk is bound with it but at about the level of the first nerve 

 of the brachial plexus separates from the vagus and enters a swelling or ganglion, 

 the middle cervical ganglion. The sympathetic cord proceeds dorsally from this 

 ganglion and lies on the ventral surface of the brachial plexus where it presents 

 two successive swellings, which together constitute the inferior cervical ganglion. 

 Observe branches from these ganglia. The sympathetic cord passes to the 

 ganglion of the first dorsal spinal nerve to which its own ganglion is fused. It 

 then proceeds as a delicate white cord across the second rib and again forms a 

 ganglion which is fused to the ganglion of the second dorsal nerve. The sympa- 

 thetic cord then passes more ventrally, lying on the side of the long neck muscles. 



