94 HISTOLOGY. 



collateral branches it may be transmitted to the motor fiber, a, causing a 

 muscle to contract involuntarily. This is the reflex path. Or the stimulus 

 from b may be conveyed to the brain along the fiber c, and be transferred 

 to commissural cells of which d is a fiber extending down the cord. This 

 also may stimulate the motor cell a, causing the muscle to contract volun- 

 tarily. 



The terms dendrite and neuraxon are of wide application. A nerve 

 cell generally has a single process which differs from the others in being 

 clear, non-granular, and sharply defined, often becoming very slender 

 soon after leaving the cell body. It may have collateral branches, usually 

 given off at right angles, but except at its termination its branches are 

 relatively few. // conducts impulses away from the cell body. This process 

 is the neuraxon. The dendrites, which develop later, appear as granular, 

 protoplasmic processes. They fork and branch freely, giving the cell a 

 great extent of exposed surface. They may serve in obtaining nutriment, 

 as well as in providing many opportunities for contact with the processes 

 of other nerve cells. Dendrites conduct impulses toward the cell body. 

 In the sensory cells of the dorsal ganglion the single peripheral fiber is a 

 dendrite of unusual form, and the fiber entering the cord is the neuraxon. 



THE SYMPATHETIC SYSTEM develops chiefly from the visceral or 

 sympathetic branches of the spinal nerves. A spinal nerve typically 

 has one such branch, extending ventrally and medially toward the aorta, 

 and ending in a clump of nerve cells (Fig. 109 E). These cells, which 

 constitute a sympathetic ganglion, are considered to have migrated along 

 the nerve bundles from the spinal ganglion, or possibly from the spinal 

 cord. They multiply by mitosis. The successive ganglia become con- 

 nected by longitudinal nerve fibers so that they form two sympathetic 

 trunks (or cords), one on either side of the vertebral column. The ganglia 

 of the sympathetic trunk are cervical, thoracic, lumbar and sacral. There 

 are only three cervical ganglia, probably because some in this region have 

 fused. In the adult the sympathetic ganglia are each usually connected 

 with the spinal nerves by two bundles of fibers, the white and gray rami 

 respectively. The smaller gray ramus is said to convey fibers from the 

 ganglion to the spinal nerve. These rami may be subdivisions of the orig- 

 inal visceral branch. 



Besides smaller branches from the three cervical ganglia to neighbor- 

 ing vessels and organs, each of these ganglia sends out a large cardiac 

 nerve, the branches of which unite to form the cardiac plexus. From 

 this plexus and the associated cardiac ganglion the fibers continue to the 

 heart muscle which they innervate. In the lower thoracic region the 

 ganglia of the sympathetic trunk send out nerve bundles which unite to 



