SPINAL NERVES IN VERTEBRATES 179 



caudalis. The first three differ in several respects from the 

 last two ; they unite in forming mixed rami ventrales and doubt- 

 less take part in the motor and sensory supply of the M. cordis 

 caudalis. Soon after this junction takes place two branches 

 from the first and a cephalic branch from each of the second and 

 third nerves pass ventrally close to the surface of the M. cordis 

 caudalis. Each of these nerves divides into a cephalic and a 

 caudal branch. These branches unite with each other in form- 

 ing a continuous longitudinal nerve, which lies close to the ceph- 

 alic ventro-lateral surface of the M. cordis caudalis, and from 

 which several branches (R.M.C.C.) enter the muscle. This 

 longitudinal nerve has connections also wdth the first ramus 

 ventralis in front of the M. cordis caudalis. It is drawn mainly 

 in black in figure 1 as if it were a motor nerve, but it doubtless 

 contains sensory fibers as well, which probably supply some 

 eight or more muscle spindles (M.S.), situated on some of the 

 large muscle bundles {L.M.B.) belonging to this muscle. The 

 motor and sensory ventral rami of the last two spinal nerves 

 passing between the myotomes and the M. cordis caudalis not 

 only remain separate throughout, but follow the contour of the 

 myotomes rather than the M. cordis caudalis and apparently 

 take no part in the innervation of the latter. The motor 

 ramus ventralis of the first spinal nerve behind the M. cordis 

 caudalis is of especial interest in this specimen because it gives 

 off a conspicuous branch (R.M.C.C), which traverses nearly 

 the entire length of the inner surface of the M. cordis caudalis 

 and probably furnishes a large part of its motor supply. 



Some variation occurs in the distribution of the spinal nerves 

 to the M. cordis caudalis in the 85 mm. embryo (fig. 3) from 

 the previous description of the 20 cm. specimen (fig. 1). There 

 is a possibility of at least an equal number of spinal nerves tak- 

 ing part in its innervation. The ventral motor and sensory 

 rami of each of the spinal nerves destined to pass between the 

 myotomes and the M. cordis caudalis remain separate through- 

 out in the 85 mm. embryo, with the exception to be noted for 

 the first nerve. In this nerve the motor and sensory ventral 

 rami unite at the level of the caudal artery and form a mixed 



