176 ESSENTIALS OF ZOOLOGY 



plate is being folded to form the neural canal. These give 

 rise to ganglia, and the cells send their axons into the nerve 

 tube and also distally. Such are sensory nerves. 



The nerves are divided into sensory and motor. The 

 sensory fibres carry stimuli from the skin and sense organs, 

 from the muscles and from the viscera. The motor fibres carry 

 stimuli outwards which produce some effect. They end in the 

 muscles or in glands. The afferent and the efferent stimuli 

 may be related in performing a simple reflex action directly, as, 

 say, through the spinal cord. And all degrees of complexity 

 between a simple reflex and the complex reflexes associated with 

 correlating paths and inhibitory paths have to be considered. 

 Not only so, but the nervous system is liable to stimuli from 

 the endocrine organs through the blood stream. 



The nerves are divided into cranial nerves and nerves 

 of the spinal cord, and these are associated also with the 

 sympathetic nerves. 



The cranial nerves are : 



I. Olfactory. The olfactory lobes are connected with 

 the sensory cells of the much-folded epithelium of the nose by 

 numerous fibres, the olfactory nerves. On their inner side 

 an accessory olfactory nerve, the nervus terminalis, may be 

 exposed with care and disclosed by sections. It is connected to 

 the brain in the wall of the lamina terminalis and is distributed 

 to the inner aspect of the nasal epithelium. It is provided with 

 a ganglion and appears to be associated with the neuropore 

 primarily 1 (fig. 89, 1'). 



II. Optic. The retina of the eye gives off many fibres which 

 leave the eye by the optic nerve. The nerves meet and fuse 

 below the thalamencephalon, forming the optic chiasma in 

 which the fibres cross from one side to the other. Beyond the 

 chiasma the nerves are absorbed in the thalami. Both these 

 nerves, it is almost unnecessary to say, are purely sensory. 



III. Oculomotor. These are motor nerves which arise in 

 the floor of the mesencephalon and pass into the orbit to supply 

 the superior, inferior, and internal recti and the inferior oblique 

 muscles of the eye. 



1 1905, Looy, Anatomischer Anzeiger, Bd. 26. 



