COORDINATION 



205 



tended to by the cerebellum. It is discriminating but not con- 

 scious. 



Finally, in the medulla, nerve fibers are in the ascendancy since 

 all nerve impulses to and from the cerebrum and cerebellum 

 and the spinal cord are trans- 

 mitted through the medulla. The 

 crura cerebri are large bundles of 

 fibers extending from the medulla 

 to the cerebrum. However, nerve 

 cells are also prominent in the 

 medulla: some of them give rise 

 to cranial nerves, and some 

 form functional groups, or centers 

 of actions for regulating the 

 respiratory and circulatory or- 

 gans, known as the respiratory 



CENTER, VASOMOTOR CENTER, etc. 



2. Cranial and Spinal Nerves 



The brain and spinal cord are, 

 as we know, protected and iso- 

 lated by a cartilaginous or bony 

 tube, formed by the skull and 

 neural arches of the vertebrae, 

 which is embedded in the muscles 

 forming the dorsal part of the 

 body wall. The sole paths of 

 nervous communication between 

 the central system and the rest of 

 the organism and its surroundings 

 are a series of pairs of cranial 

 and spinal nerves. These arise 

 at fairly regular intervals from 

 one end of the brain and cord to 

 the other, and pass out through 

 openings in the skull and between 

 or through the vertebrae to con- 

 stitute the peripheral nervous 

 system. (Figs. 109, 142.) 



Fig. 142. — Ventral view of the 

 nervous system of the Frog. Br, sec- 

 ond and third spinal nerves (brachial 

 plexus) ; Js, sciatic nerve leading from 

 the sciatic plexus; O, eye; 01, olfac- 

 tory nerve; Op, optic nerve; Sg 1-10, 

 ten ganglia of autonomic system; 

 Spn 1, first spinal nerve; Sp 4, fourth 

 spinal nerve; Vg, trigeminal ganglion; 

 Xg, ganglion of 10th cranial nerve, 

 or vagus. (From Ecker.) 



