238 VERTEBRATE ZOOLOGY 



lung into two; the anterior lobe of the left lung is subdivided 

 into two lobes. A branch of the bronchus penetrates into each 

 lobe. Dissect a branch of the bronchus and follow it as far as 

 possible into the lung. 



Exercise 30. Make a drawing of the ventral surface of the lungs, the 

 bronchi, and the trachea. 



The Nervous System. This system is composed of (1) the 

 central nervous system, or the brain and the spinal cord; (2) the 

 peripheral nervous system, composed of (a) the paired cranial 

 and spinal nerves and (b) the sympathetic nervous system; and 

 (3) the special sense organs. 



We shall first study the spinal nerves and the sympathetic 

 system. 



Thirty-eight pairs of spinal nerves are present, of which eight 

 are cervical, thirteen thoracic, seven lumbar, three sacral, and 

 usually seven or eight caudal. With the exception of the first 

 two pairs, all the spinal nerves pass from the neural canal through 

 openings between the vertebrae, called intervertebral foramina. 

 The first pair passes through a pair of foramina in the dorsal arch 

 of the atlas, and the second pair between the arches of the atlas 

 and axis. Each nerve arises from the spinal cord by two roots, 

 which join immediately outside the intervertebral foramen. Of 

 these roots the dorsal one bears a ganglion and is sensory in func- 

 tion, while the ventral root is motor. Immediately after its forma- 

 tion by the meeting of these roots, the spinal nerve divides into 

 two branches, a smaller, dorsal and a larger, ventral ramus. 



At each end of the trunk the spinal nerves unite and form a 

 complicated network called a nerve plexus. That at the anterior 

 end is the brachial plexus. It lies in the axillary region near the 

 shoulder, and is formed of the ventral rami of the fifth, sixth, 

 seventh, and eighth cervical nerves and the first thoracic nerve ; the 

 nerves which arise from it go to the arm and shoulder. 



Identify the roots of these five nerves. That of the fifth cervical 

 is the smallest ; the others are large and conspicuous. Carefully 

 free the plexus of the surrounding tissue and follow the nerves 

 which spring from it. The arrangement of the network is variable, 



