26 MORPHOLOGICAL TYPES 



important to notice that the roof of the brain in Petromyzon 

 is very thin and membranous ; with the exception of the 

 transverse commissures in the forebrain, the optic lobes, and 

 the cerebellum, it contains scarcely any nerve-cells at all. 



Although at first sight the brain differs considerably from 

 the more posterior part of the nerve- tube or spinal cord, it is 

 easy to see how it was derived from the latter. 



In each segment on each side there is a ventral nerve 

 supplying the segmented muscles formed from the myotomes 

 of the vertebral plate (somatic muscles), and a dorsal nerve 

 supplying the sense-organs. In the gill-region, the dorsal 

 nerves also supply the muscles formed from the unsegmented 

 lateral plate (visceral or splanchnic muscles). In the region 

 of the trunk these segmental nerves are called spinal nerves, 

 those which emerge from the brain are called cranial nerves. 



Nerves. — The ventral nerves of the first three segments 

 innervate the muscles which actuate the eyeball. They are 

 respectively the oculomotor, trochlear, and abducens. The 

 dorsal nerves corresponding to them are the profundus, 

 trigeminal, and facial ; the auditory nerve is a branch of the 

 facial. The ventral root of the 4th segment supplies the 

 most anterior complete myotome, and its corresponding 

 dorsal nerve, the glossopharyngeal, passes down in the arch 

 behind the first gill-slit (and in front of the second). The 

 next dorsal nerve, the vagus, is a composite one, formed by 

 the aggregation of portions of several other posterior dorsal 

 nerves. It sends a branch down behind each of the remaining 

 gill-slits, as well as to the " lateral-line " organs (see p. 38), 

 and to the heart and gut. The ventral roots corresponding 

 to the vagus supply the 5th and following myotomes, and the 

 muscles beneath the gills. The muscles of the tongue are 

 supplied by the trigeminal nerve. 



In Amphioxus, the dorsal or sensory nerves are formed of 

 fibres produced inwards from the sensory cells themselves. 

 In Petromyzon and all higher forms this method of formation 

 applies only to the olfactory nerves. All the other sensory 

 nerves are formed in a different way. There are special nerve- 

 cells which send one fibre to the sensory cell and another 



