THE HIXDBRAIN 359 



bands on the surface of the cerebral peduncles, which have 

 been reduced by the ending of many fibres in the pons. 



(h) The trapezoid body (corpus trapezoideum) is another, 

 smaller, superficial transverse band just behind the pons, 

 and is part of the auditory pathway. It lies in the angle 

 formed by the lateral margin of the pyramid with the 

 posterior border of the pons, its fibres passing through 

 the deeper part of the pyramid (Fig. 123) so that they 

 are concealed by the latter. 



(i) The fifth cranial, or trigeminal nerve (n. trigeminus), arises 

 by two roots, a larger sensory root, the portio major, and 

 a smaller motor root, the portio minor. The two parts 

 appear at the lateral border of the pons, whence they are 

 directed forward. 



The portio major is the common trunk of the ophthalmic, maxillary, 

 and mandibular nerves, providing for the cutaneous sensibility of most 

 of the head. The portio minor (motor, to muscles of mastication) joins 

 the mandibular, so that the latter becomes a mixed nerve. The cut end 

 of the portio major may be identified on the cranial wall and traced 

 forward into the semilunar ganglion, the latter lying in a depression 

 at the anterior ventral end of the petrosal bone. 



(j) The sixth cranial, or abducent nerve (n. abducens), which 

 controls the lateral rectus muscle of the eye, is a slender 

 cord arising by several very delicate rootlets along the 

 lateral edge of the anterior end of the pyramid. 



(k) The seventh cranial, or facial nerve (n. facialis), and the 

 eighth, or acoustic nerve (n. acusticus), appear to rise from 

 the lateral margin of the trapezoid body. 



The two nerves are closely associated, the former being slightly 

 anterior in position. Its chief portion is the motor root which controls 

 the facial muscles. In addition the nerve receives a sensory filament, 

 the portio intermedia or intermediate nerve, bearing gustatory and 

 related impulses. The eighth nerve is purely sensory but comprises 

 two portions conveying respectively auditory and equilibratory im- 

 pulses from the corresponding parts of the internal ear. The trapezoid 

 body is composed of fibres transmitting impulses from the auditory 

 portion of the nerve, which-^ fibres decussate and eventually reach the 

 inferior colliculus and the medial geniculate body. 



(l) The glossopharyngeus, vagus, and accessorius arise by 

 several roots arranged in a linear series along the lateral 



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