153 



from the second spinal sympathetic ganglion {com. ii.), 

 and sends a mixed bundle to the H. ventralis of the first 

 spinal nerve as above described {r. v. 2^). Just at about 

 the same place it fuses completely with the R. ventralis of 

 the third spinal nerve, but the above anastomosis is 

 derived from the R. ventralis 2, and contains no fibres 

 from 3. The compound trunk (r. v. 2 + 3), after giving off 

 a small branch to the inner surface of the clavicle (not 

 shown in the chart), courses forwards and downwards, and 

 gives off below two motor branches which pass at first 

 downwards and then forwards to supply the ventral 

 musculature. They anastomose with each other close to 

 their origin, and the posterior one (r. v. 2 + 3^) also anasto- 

 moses below with the R. ventralis of the fourth spinal 

 nerve. The remainder of the trunk then splits behind 

 into two — a small motor nerve and a larger mostly sensory 

 nerve. The latter (r. v. 2 + 3") is continued backwards on 

 to the ventral portion of the pectoral fin. 



The Third Spinal Nerve. 



This spinal nerve has the usual two roots {d. 3 and 

 V 3), and a single very large extra-vertebral ganglion 

 [g.. 3). Each root has a separate foramen in the neural 

 arch of the second vertebra (fig. 17). The sensory R. 

 communicans (r. com. 3) contains a few motor fibres, which 

 are liberated as a small bundle for the dorsal musculature 

 (not shown in the chart). It fuses with the R. spinosus 2, 

 as above described. The motor R. spinosus (r. sp. 3), 

 after rising internal to the ganglion, courses at first back- 

 wards, fuses with the R. communicans 4, and then turns 

 forwards as a mixed nerve high over the roof of the skull 

 and very close to the middle line. It gives off below the 

 motor nerve to the dorsal musculature just like the pre- 

 ceding nerve. 



