208 



ANIMAL BIOLOGY 



root only motor (efferent) fibers. This condition is preserved by 

 the spinal nerves of higher forms since each arises by two roots. 

 But some of the cranial nerves, in response to the profound modi- 

 fications that have been wrought in the 

 head region, have only one root, and so 

 are either solely sensory, as those to the 

 sense organs, or only motor, as those 

 innervating the muscles which move 

 the eye. (Fig. 143.) 



Many nerve impulses set up by sen- 

 sory stimuli are, in part, shunted di- 

 rectly from sensory to motor nerve 

 paths in the spinal cord itself. One 

 removes his finger from the pin-point 

 before he is conscious of the prick. 

 Thus so-called reflex arcs bring 

 about the multitude of reflexes 

 which relieve the brain of much unnec- 

 essary labor, and are the basis of the 

 behavior of animals. Many reflexes 

 apparently are inherited, but others, 

 known as conditioned reflexes, are 

 established as the result of experience. 

 (Figs. 139, 143.) 



3. Autonomic System 



So far we have considered the central 

 system- -the brain and spinal cord; 

 and its lines of communication with 

 the body as a whole, the peripheral 



g — . 



Fig. 144. — Diagram of a 

 section (highly magnified) of 

 the wall of the intestine of a 

 Vertebrate to show its intrin- 

 sic nervous organization 

 which brings about the move- 

 ments of the tube. The two 

 plexuses consist essentially of 

 simple neurons arranged as 

 nerve nets, a, food absorbing 

 surface of the intestine; b, 



mucous layer; c, plexus of system — the cranial and spinal nerves. 



neurons (submucous); d, cir- 

 cular muscle; e, plexus of 

 neurons (myenteric); /, lon- 

 gitudinal muscle; g, serous 

 layer. (From Parker, after 

 Lewis.) 



In point of fact, however, the periph- 

 eral system gives rise to an auxiliary 

 series of ganglia and nerves which are 

 charged with the regulation of prac- 

 tically all of the functions of the body 

 that are not under voluntary control, such as the circulatory 

 system and alimentary canal. This autonomic system in the 

 higher \ ertebrates consists essentially of a double nerve chain, 

 situated chiefly in the body cavity just ventral to the verte- 



