260 



PRACTICAL ZOOLOGY 



of nervous activity. The apparatus required for a simple reflex 

 in the body of a frog is shown in Figure 148. A sensory cell 

 lying at the surface of the body (s) sends a fiber (s.f) into the 

 spinal cord, where it branches out; these branches are in physio- 

 logical continuity with branches from a motor cell (v.c) lying 

 in the ganglion of the spinal cord. The motor cell (v.c) sends 

 fibers (m.f) into a reacting organ, such as a muscle (M). These 



FIG. 148. Diagram of the spinal cord showing the paths taken by nervous 

 impulses. The direction of the impulses is indicated by arrows. 



c.c, central canal ; col, collateral fibers ; c.cort, cell in the cerebral cortex ; 

 c.g, smaller cerebral cell; d.c, cells in 'dorsal horn of gray matter; d.r, dorsal 

 root; g, ganglion of dorsal root; g.C, ganglion cell in dorsal ganglion; g.m, 

 gray matter; M, muscle; m.c, cell in medulla oblongata; m.f, motor fiber; 

 S, skin; s.f, sensory fiber; sp.c, spinal cord; v.c, cells in ventral horn of gray 

 matter; v.r, ventral root; w.m, white matter. (After Parker.) 



fibers extending to the reacting organ are called motor fibers 

 (m.J) ; those leading to the spinal cord are termed sensory fibers 

 (s.f). The sensory cell or receptor receives the stimulus and 

 produces the nerve impulse; the motor cell, the adjuster, receives, 

 directs, and modifies the impulse; and the muscle or other organ 

 stimulated to activity is the e/ector. Within the spinal cord are 

 association cells (c.d) whose fibers serve to connect structures 

 within one ganglion or two succeeding ganglia. 



The SYMPATHETIC SYSTEM consists of two principal trunks, 

 which lie one on either side of the vertebral column. The nerves 



