Johnston, Morphology of the Head. 229 



medulla oblongata and between dorsal general cutaneous roots 

 and ventral somatic motor roots. The accessory illustrates the 

 method by which the collection of motor roots may take place. 

 The shifting of motor roots obviously can not be due directly 

 to such peripheral causes as the crowding together of rami 

 which explained the collection of sensory nerves. The motor 

 roots must have been collected under the influence of the sen- 

 sory components and so indirectly on account of the same 

 causes. As the motor fibers grow out from their nuclei in the 

 brain they must follow some path of low resistance in travelling 

 to their muscles. Since the motor fibers develop late they find 

 such a path already provided in the near-by sensory root. 

 The motor fibers follow this and a mixed trunk is formed. 

 When the sensory fibers of a given root shift their course be- 

 tween the epibranchial ganglia and the brain to the root next 

 cephalad, the motor fibers on issuing from the brain find no 

 path in that segment but must turn forward to the next cephalic 

 sensory root and follow it. As this goes on gradually from seg- 

 ment to segment there are formed a number of roots emerging 

 from the cord or brain caudal to the complex and running along 

 the side of the brain to join it. This is the description of the 

 accessory roots. The more cephalic roots have become more 

 compactly arranged and their fibers run backward within the 

 brain, parallel with the somatic motor fasciculus, to their nuclei 

 of origin. This is the description of the vagus motor roots. 

 Both vagus and accessory motor roots are formed by the same 

 process and constitute a graduated series. 



This is a very brief summary of views on a complex region 

 to which hundreds of pages have been devoted by observers in 

 this special field. But even from this it must be evident that 

 the region between the head and trunk is one of transition and 

 the attempt to set a caudal limit to the head which shall be 

 valid for all vertebrates is a fruitless one. Reduction and loss 

 of myotomes with loss of somatic motor nerves, reduction of cu- 

 taneous area and nerves, growth and expansion of gills followed 

 by their reduction and loss, shifting and reduction of visceral 

 sensory and motor nerves, shifting of skeletal elements, fusion 



