CRANIAL NERVES l8l 



All of the components of a spinal nerve (somatic sensory, somatic 

 motor, visceral sensory and visceral motor) are distributed among the 

 cranial nerves, and in addition two other components occur — a 

 lateraUs system and the nerves of special sense, giving a total of six 

 different kinds of conducting tracts. Regarding these a few words 

 may be said. The somatic sensory nerves (called in the head the 

 general cutaneous) terminate in the skin, either as free nerve ends or 

 in special tactile sense organs. The visceral sensory fibres end in 

 taste organs, usually inside the mouth, but in teleostome fishes these 

 may be distributed over the surface of the body. The lateralis 

 elements are sensory and terminate in pecuUar collections of sense 

 cells, known as sense hillocks or neuromasts in the ear and in the 

 lateral line organs (p. 195) of the lower (branchiate) ichthyopsida. 



Taking components and other facts into consideration, the cranial 

 nerves may be arranged in several groups. Some of these contain 

 but a single component, others are more mixed in character. The 

 constitution of these latter may be seen in general outlines in fig. 

 189; further details concerning them should be sought in special 

 papers. These groups are as follows: 



1. Nerves of special sense — olfactory and optic. These arise 

 from the primitive fore-brain and go respectively to the nose and eye. 

 In several points they are without parallel among the spinal nerves. 



2. The eye-muscle nerves — oculomotor, trochlear, and abducens 

 — are somatic motor (with some visceral sensory and motor fibres in 

 the oculomotor). These control the muscles of the eye (p. 133). 

 The hypoglossal is also in the purely somatic motor group, while the 

 accessory is visceral motor. 



3. The acustico -lateralis system. This includes the acustic 

 nerve of all vertebrates, and in the branchiate ichthyopsidans, por- 

 tions of the seventh and tenth nerves which are distributed to the 

 pecuHar sense organs of the lateral Unes of the head and body. These 

 nerves have a separate centre in the upper anterior end of the 

 medulla oblongata. 



4. The fifth, seventh, ninth and tenth nerves, which approach 

 more nearly to a spinal nerve in character. These arise from the 

 medulla, each has only dorsal roots, is gangUonated and contains at 

 least somatic sensory (usually called general cutaneous in the head) 

 and visceral sensory and motor components. In addition, as indi- 

 cated under 3, the seventh and tenth may also include lateralis ele- 

 ments. Of these the ninth (fig. 190, IX) is the simplest. It arises 



