SEA-FISHERIES LABORATORY. 295 



lateralis accessories is absent in the Plaice, and as it 

 cannot anastomose with the R. . spinosus, this nerve is not 

 an actual R. commimicans in the Plaice. 



2. R. medius, b. (r. m. &.). — Leaves the ganglion as 

 two distinct strands — an anterior sensory and a posterior 

 motor. The sensory section is small and passes outwards 

 and forwards to the skin in the region of the lateral 

 sensory canal. The motor section arises from the fibres 

 of the fused first and second ventral roots which have 

 passed underneath the intracranial ganglion, traversed 

 the main foramen lying underneath motor fibres from the 

 third ventral root, and perforated the extracranial 

 ganglion. Immediately it leaves the latter ganglion it 

 gives off a small anterior branch, whilst the rest of the 

 nerve passes laterally backwards and breaks up largely in 

 the dorsal musculature. 



3. R. medius, c. (r. m. c). — A large mostly motor 

 nerve arising almost entirely from the third ventral root. 

 The fibres pass through the main foramen, and perforate 

 and leave the extracranial ganglion at its anterior aspect. 

 Directly it leaves the ganglion, it gives off above a small 

 sensory thread, which passes outwards and backwards 

 towards the lateral sensory canal. The remainder and 

 larger part of the nerve courses downwards, outwards and 

 backwards in the dorsal musculature, in which most of its 

 fibres break up, except also a few that pass outwards 

 towards the skin. 



The Second Spinal Nerve. 

 The second spinal nerve has a dorsal mostly sensory 

 (d. 2) and a ventral motor (v. 2) root, and two extra- 

 vertebral ganglia — a large dorsal ganglion (g. d. 2) and 

 a smaller ventral ganglion (g. v. 2). The dorsal root 

 passes backwards at once into the dorsal ganglion, but the 



