THE PIGEON 181 



partly ossified arytenoid cartilages. Remove the delicate muscles 

 and the mucous membrane from the larynx and note the shape 

 of the arytenoids. 



Back of these cartilages are the procricoid and cricoid cartilages, 

 which are modified tracheal rings. A thyroid cartilage is not 

 present in birds. The cricoid cartilage, often wrongly called the 

 thyroid, is a partly ossified ring, which is wide ventrally and in- 

 complete on the dorsal side. Between its dorsal ends lies a small 

 median plate, the procricoid, sometimes wrongly called the cricoid. 

 The dorsal ends of the two arytenoids articulate with the procri- 

 coid ; the ventral ends are free. 



Exercise 20. Draw a lateral view of the larynx on a scale of 2 or 3. 

 Exercise 21. Draw an outline of the oesophagus with the crop. 



The Nervous System. This consists of (1) the central nervous 

 system, which is made up of the brain and the spinal cord ; (2) the 

 peripheral nervous system, which includes the paired cranial and 

 spinal nerves and the sympathetic nervous system; and (3) the 

 special sense organs. 



We shall first study the spinal nerves and the sympathetic nerv- 

 ous system. Remove from the body what remains of the kidneys, 

 but be careful not to injure the nerves that lie beneath them. 



The spinal nerves are those which place the spinal cord in com- 

 munication with the muscles and skin of the trunk, neck, and 

 limbs. They join the spinal cord in pairs, a pair lying between 

 each two vertebrae in the intervertebral foramina. A single spinal 

 nerve is formed of two roots : a dorsal root, the fibers of which are 

 sensory in function, and a ventral root, which is motor. On the 

 former is a large swelling, the spinal ganglion. The two roots 

 meet outside the intervertebral foramen, at the distal end of 

 the ganglion. 



Observe the spinal nerves of the trunk ; they will be seen to be 

 white cords lying on the dorsal wall of the body cavity between 

 and parallel with the ribs. At the point where the neck joins the 

 trunk, note the prominent retractor muscles, which connect the 

 ventral and lateral surfaces of the former with the ventral surface 

 of the thoracic vertebrae. Cut away these muscles ; lying on them 



