30 Anatomy of the Nervous System 



making up seven out of the complete set of eight components. 

 The general somatic afferent system is represented mainly in 

 the trigeminal nerve, and a small number of fibres of this 

 system occur in the facial/ glossopharyngeal, and vagus 

 nerves. It is also present, in the form of proprioceptive fibres 

 for muscle sense in the eye muscles, in the oculomotor, troch- 

 lear, and abducent nerves. 



All the general somatic afferent fibres which carry extero- 

 ceptive impulses from the head region, no matter what their 

 peripheral course may be, end centrally in a single nucleus 

 of gray matter in the medulla oblongata. Since the great 

 majority of such fibres are found in the trigeminal nerve, this 

 centre has received the name of trigeminal nucleus. Upon 

 entering the brain, most of the afferent fibres bifurcate, 

 forming ascending and descending branches, of which the 

 latter are the longer. A considerable number of unmyelinated 

 and small myelinated fibres, however, have recently been 

 found in the rat and other mammals to descend without 

 bifurcating. They are believed to be pain fibres, as this sense 

 is absent from the chief nucleus. The fibres are grouped 

 together in a compact bundle running along the dorso-lateral 

 aspect of the medulla in a superficial position. This spinal V 

 root {radix spinalis sen descendens trigemini) extends down- 

 wards from the place of entry of the trigeminal nerve to the 

 beginning of the spinal cord, where it becomes continuous 

 with the tract of Lissauer (Pis. VL-XI.). Some of the ascend- 

 ing branches of the sensory fibres pass upwards beyond the 

 level of the entry of the fifth nerve before ending, forming 

 an ascending extension of the bundle for a short distance. 

 The number of fibres in this root and, consequently, the size 

 of the fasciculus as a whole vary directly with the degree of 

 development of general somatic sensibility in the head region. 



^The presence of cutaneous sensory fibres in the facial nerve has been 

 demonstrated in the mouse, and in various lower vertebrates. They 

 probably occur in other mammals also. 



