370 COMPARATIVE ZOOLOGY OF CHORDATES 



muscles and glands, and forms part of the autonomic (para- 

 sympathetic) system. The fibres run through the oculomotor, 

 facial, glossopharyngeal, and vagus nerves. The centre of 

 origin of the visceral efferent neurons is for the most part in 

 the medulla oblongata, beneath the visceral lobe. 



The somatic efferent system is concerned with the innerva- 

 tion of striated voluntary muscles derived from the segmented 

 myotomes. In the head these are represented by the muscles 

 which move the eyeballs, and the hypoglossal muscles. This 

 component is, therefore, to be found in the oculomotor, 

 trochlear, abducens, and hypoglossal nerves. The centres of 

 the oculomotor and trochlear are in the mid-brain, those of 

 the abducens and hypoglossal are in the medulla oblongata. 



Fig. 170, G. — The proprioceptive fibres of the general somatic sensory 

 component. 



It may be noticed that the arrangement in the medulla 

 oblongata of the centres concerned with the various components, 

 is similar in a general way to that which holds in the spinal 

 cord. The medulla is the least specialised portion of the brain. 



The eyes themselves are part of the brain, and therefore the 

 optic nerve is not an ordinary nerve. Its fibres are strictly 

 intra-cerebral throughout their course. They run through 

 the optic chiasma and end in the roof of the midbrain, which 

 is enlarged to form the optic lobes, or " eye-brain." 



The nasal sacs are lined by sensory epithelium, the cells 

 of which produce axons growing back into the end-brain. 

 The latter becomes enlarged to form the olfactory lobes or 

 " nose-brain." 



Expressed in tabular form, the component nerve-systems 

 are as follows : — 



