RAT, A REPRESENTATIVE MAMMAL 139 



A summary of pertinent information concerning this group of 

 nerves is organized into the accompanying table. 



The paired spinal nerves are connected to the spinal cord by a 

 dorsal (sensory) root and a ventral (motor) root. The former trans- 

 mits impulses to the cord, and the latter consists of fibers carrying 

 motor impulses away from the cord. A small ganglion is located in 

 each dorsal root. The two roots forming each nerve join each other 

 while still within the neural canal of the vertebral column, and the 

 nerve then emerges through the intervertebral foramen of that level. 

 Immediately outside the intervertebral foramen each nerve gives off 

 a dorsal branch (ramus) to the muscles of the back, the main ventral 

 branch, and a small connecting bundle (ramus communicans) to the 

 autonomic system. In general, a pair of spinal nerves leaves the 

 spinal cord at each meeting of adjacent vertebrae for most of the 

 length of the vertebral column. In certain regions these nerves anas- 

 tomose (join in a network), forming plexuses. In the neck region 

 is the cervical plexus; in the region of the shoulder and forelimb, the 

 brachial plexus; in the region of the loins, the lumbar plexus; and in 

 the region of the sacrum, the sacral plexus. 



Sense Orguns. — The organs of sense, or receptors, and specialized 

 peripheral endings of the sensory nerves, are constructed to be par- 

 ticularly efficient in receiving a certain type of stimuli. The eyes, 

 ears, olfactory membrane of the nose, taste buds, pressure and 

 tactile corpuscles are all endings of this kind. The stimuli for the 

 eye are in the form of light waves ; those of the ear are ether vibra- 

 tions ; those of taste and smell are chemical ; and those of touch are 

 pressure and contact. These organs and their function will be dis- 

 cussed in the chapter on Physiology. 



Autonomic Nervous System. — The mere sight of food to a hungry 

 person causes the flow of both saliva and gastric juice. This activ- 

 ity occurs without conscious control and is one of the multitude of 

 examples of the functioning of the autonomic portion of the nervous 

 system. The principal parts composing this system include two 

 trunks or chains of ganglia, one at each side of the vertebral 

 column on the dorsal wall of the coelome. The anterior extremity 

 of each trunk is the superior cervical ganglion near the angle of the 

 jaw and covered by the submaxillary gland. In each trunk there are 

 cervical (or cranial) ganglia with fibers extending to the ciliary 

 body in the eye, salivary glands, heart, bronchi, stomach, intestines, 



