COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 319 



carefully clearing this away the dorsal ganglion of the spinal nerve will be found 

 imbedded in it. It is a rounded brownish body from which spring two nerves, 

 the ventral ramus just followed and the smaller dorsal ramus which supplies the 

 epaxial muscles. 



2. The limb plexi. — The ventral rami of the spinal nerves form a plexus 

 for each limb, the motor nerves for the limb muscles arising from this plexus. 

 The brachial plexus is located as follows. Make a cut in the ventral body wall 

 just medial to the base of the fore limb. Separate the pectoral and shoulder 

 muscles from the sternohyoid muscle. The brachial plexus is then easily seen 

 running posterior to the scapula. It consists of the ventral rami of three spinal 

 nerves (3, 4, and 5) which have cross-connections with each other. Beyond the 

 plexus, nerves proceed into the fore limb. 



The lumbosacral plexus is located by cutting through the skin and both layers 

 of oblique muscles longitudinally just dorsal to the base of the hind limb. On 

 separating the oblique from the transverse muscle layers the plexus is exposed. 

 It consists of the ventral rami of three (sometimes four) spinal nerves. The 

 anterior one of the three gives off a slight branch to the nerve next posterior to 

 it, sometimes receives a contribution from the ramus anterior to it, and as the 

 crural nerve, passes into the limb. The middle of the three nerves is much the 

 stoutest, and after receiving contributions from the nerve next posterior to it 

 enters the limb as the ischiadic (sciatic) nerve. 



3. The sense organs of the head. — Owing to the small size of the eye and its 

 similarity to that of other vertebrates it will not be investigated. 



a) Lateral line system: Lateral line sense organs are present in Necturus but 

 impractical to find in gross dissection. They are situated along lines similar to 

 those of fishes. 



b) Nose: Probe into the external naris and follow your probe with the cut, 

 opening the entire nasal passage to the internal naris. Note the folds or olfactory 

 lamellae in the interior of the passage. Unlike the condition in fishes, the 

 olfactory sac opens into the oral cavity and has both olfactory and respiratory 

 functions. 



c) Ear: The ear as in fishes consists of the internal ear only, imbedded in 

 the otic region of the skull. Expose the dorsal surface of the skull by cleaning 

 away the muscles. Locate the otic capsules, one at either side of the posterior 

 end of the skull. Cautiously shave away the cartilage here and locate the three 

 semicircular ducts and vestibule as done in the dogfish. There are three of the 

 former — anterior vertical, posterior vertical, and horizontal — each with an ampulla, 

 all arranged exactly as in the dogfish. The vestibule is divided indistinctly into 

 a dorsal utriculus, from which the ducts spring, and a ventral sacculus. The 

 latter contains a crystalline mass, the otolith. 



4. The dorsal aspect of the brain.— Remove the roof of the skull. This is 

 best done by stripping it off in slivers with a knife. After the brain is revealed 



