574 TEXTBOOK OF ZOOLOGY 



trunks is connected with all of the spinal nerves on the same side 

 of the body by a number of ganglionic enlargements. The usual 

 sensory and motor functions are carried on by these systems. The 

 autonomic system helps to regulate involuntary reactions. 



The special sense organs of the head are the eyes and the ears. 

 Each eyelall reposes in an orbital cavity, and is attached to the walls 

 of the cavity by voluntary muscles which serve to move it in all direc- 

 tions. The innervation is from the optic lobes and other parts of the 

 brain by way of the optic nerves. The structure of the eye itself is 

 similar to that of other vertebrates. 



The ear consists of two principal parts: the middle ear, or tym- 

 panum, and the internal ear, or membranous labyrinth. The cavity 

 is closed outwardly by the tympanic membrane. It communicates 

 with the mouth cavity by means of the Eustachian passage. A small 

 rod of bone and cartilage, the columella, stretches across the cavity 

 from the tympanic membrane and is fixed internally into the mem- 

 brane covering the fenestra ovalis. The internal ear is enclosed by 

 the bones of the auditory region. 



The Skeletal System 



The reptilian skeleton retains many features of the amphibian 

 skeleton and forecasts many features found in birds and mammals. 



The divisions of the skeleton of the horned lizard are the skull, 

 the vertebral column, ribs, sternum, the limb girdles, and limbs. 



The skull proper is triangular and somewhat pyramidal in form. 

 The base of the pyramid is represented by the posterior aspect of the 

 skull, and the apex is represented by the snout. It is more com- 

 pletely ossified than in amphibians and is composed of a greater 

 number of bones. There are sharp protuberances and excrescences 

 that form the skeletal support of the "horns" of horned lizards. 

 Were these magnified to the size of dinosaurs, they would be equally 

 imposing. The skull articulates posteriorly with the atlas, or first 

 cervical vertebra. 



The vertebral column consists of several distinct regions and a 

 number of vertebrae. There is a cervical region composed of seven 

 vertebrae which form the skeleton of the neck. A thoracicolumbar 

 region consists of fourteen vertebrae and with the ribs and sternum 



