STRUCTURE OF THE VERTEBRATES 277 



located in the ventral portion of the cord. Dendrites receive 

 the stimulus and a motor impulse is carried out over the axone 

 to a muscle, gland, or other effector. This is the simplest type of 

 nerve reaction. It is doubtful if a two-neurone arc is possible; 

 and in the normal animal an arc as simple as the one described 

 rarely occurs. Every sensory stimulus reaching the cord is car- 

 ried to the brain, and numerous pathways are usually stimulated. 

 The anatomical connection between two neurones is the 

 synapse. There is no evidence that the neurofibrils are in actual 

 contact, the experimental evidence indicating that there is a 

 definite space between. The gap apparently causes the impulse 

 to travel in one direction, for experiments have shown that nerve 

 fibers carry impulses in either direction with equal facility. 



C. The Cord axd Spinal Nerves 



The spinal cord is more primitive than the brain and will be 

 described first on account of its simplicity. In dissection the 

 spinal cord of the primitive vertebrate appears as a tapering 

 structure with two enlargements; one in the neck, the cervical 

 enlargement, and a posterior one near the pelvic limbs, the 

 sacral enlargement. Except in the most primitive types, the 

 cord is divided both dorsally and ventrally by deep fissures. In 

 transverse section the cord of the higher vertebrates is approx- 

 imately circular, and almost divided into two halves. The fis- 

 sures are caused by the unequal growth of the embryonic tube, 

 the cord enlarging laterally and not dorso-ventrally. In the 

 center is the neurocoel, surrounded by the tissue and fibers 

 connecting the right and left sides. The peripheral region of the 

 cord is white substance; and centrally is an H-shaped region of 

 gray substance, the neurocoel being in the center of the cross 

 bar. 



In the primitive vertebrate the spinal nerves leave the cord as 

 paired, metameric structures. Each has a dorsal sensory root 

 with its dorsal ganglion near the cord, and a ventral motor root. 

 In Amphioxus and some cyclostomes (Petromyzon and others) 

 these roots do not unite, but pass independently to the tissues of 

 the body. In all other vertebrates the roots coalesce to form a 

 single spinal nerve. 



