XIX] CRANIAL NERVES 425 



bound up a certain distance with the long peripheral processes of the 

 sense-cells which constitute the spinal ganglia, so as to form a com- 

 pound sensory-motor nerve, which is then said to have a dorsal 

 sensory and a ventral motor root. 



In Craniata there exist below the spinal ganglia another series 

 termed the sympathetic ganglia. These ganglia retain their 

 connection with the spinal cord by nerves called the rami com- 

 municantes, and from them there are given off motor nerves to 

 the smooth visceral muscles encircling the blood-vessels. Successive 

 sympathetic ganglia are connected by a longitudinal commissure, 

 and so there is a chain of sympathetic ganglia on each side of the 

 spinal cord. The purpose of this commissure seems to be to distribute 

 the nervous impulse proceeding from the spinal cord over a con- 

 siderable area and to prevent too acute a contraction occurring in any 

 one part of a blood-vessel. They must be regarded as portions 

 separated from the ventral or motor part of the spinal cord. 



In the higher Craniata another series of ganglia (the visceral 

 ganglia) are situated on the mesentery. These are connected with 

 the sympathetic ganglia and send nerves to the muscles of the gut, 

 the penstalsis of which they control. It is usual to reckon ten pairs 

 of nerves as appertaining to the brain. The first or olfactory pair 

 are formed by a large number of nerve fibres connecting the olfactory 

 lobes of the brain with the epithelium of the nasal sacs (Fig. 207). 

 The second or optic nerve is formed by nerve-fibres growing along 

 the stalk uniting the primary optic vesicle with the brain (Fig. 211). 

 The nerve fibrils which run in this stalk go mainly but not entirely 

 to the opposite side of the brain, each side of the brain receiving 

 impulses from both eyes. Thus in the floor of the thalamencephalon, 

 or primitive fore-brain, there is a crossing of fibres proceeding from 

 the two eyes. This part of the floor becomes nipped off by grooves 

 from the rest, and is known as the optic chiasma. The chiasma 

 is connected with the combination of the stimuli received by the 

 two eyes so as to produce a reflex impulse which is transmitted to 

 the eye-muscles and causes them to direct both eyes towards the 

 same point of the compass and thus to be focused on the same 

 object. In people who squint the over development of one of the 

 muscles of one eye prevents this purpose being attained. The third 

 or motor oculi, the fourth or patheticus, and the sixth or 

 abducens nerves are motor nerves, supplying the eye muscles 

 derived from the head cavity, the mandibular cavity and the first 

 myotome respectively (see p. 437). 



