RESPONSIVENESS IN VERTEBRATES 



The Sense Organs. The sense organs or receptors of vertebrates are 

 regions in which certain cells exhibit in a conspicuous manner the capacity 

 of responsiveness in connection with particular changes in the environment. 

 Organs of special sense, such as the eyes, ears, olfactory epithelium, and 

 taste buds, as well as receptors for touch, temperature, and pain are located 

 on or near the surface of the body. In addition to the peripheral sense 

 organs, there are receptors in muscles, joints, and internal organs. It is not 

 our purpose to discuss the structure of any of these organs but to call attention 

 to their relations with the peripheral and central nervous systems. For ex- 

 ample, the olfactory nerve extends from the olfactory epithelium of the nasal 

 cavity to the brain, and the optic nerve passes from the retina of the eye to 

 the brain (Figs. 4.2 and 4.3). The auditory nerve connects the sensory 

 epithelium of the inner ear with the brain (Figs. 4.4, 4.16, and 4.17). 



The Skeletal and Muscular Systems. The body of vertebrate animals 

 is made up of the head and trunk; in some, a neck and tail are present as ex- 

 tensions of the trunk region. Also, in the region of the trunk there are two 

 pairs of appendages, fins in aquatic forms and limbs in the terrestrial ones. 

 The skeleton which supports the body of a vertebrate is internal and called 

 an endoskeleton. This skeleton serves as a protective case for the central 

 nervous system and as a rigid framework for attachment of the muscles. 



Skull 



Fig. 4.5. Axial skeleton of man, 

 in lateral view, with skull and 

 vertebral column cut in median 

 section. (Redrawn from T. 

 Hough and W. T. Sedgwick, 

 Human Mechanism, copyright 

 1918 by Ginn and Co., printed 

 by permission.) 



Sternum 



Ribs 



Pelvic 

 girdle 



>• Thoracic 



- Lumbar 



Sacral 



Coccyx 



Vertebral 

 column 



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