THE SOMATIC MOTOR DIVISION. 197 



of tactile impulses which are brought into the spinal cord by the 

 cutaneous fibers of the dorsal roots. Collateral branches of these 

 fibers carry the impulses directly to the motor cells. A somewhat 

 more complex course for similar impulses is illustrated in the 

 relations of the acusticum and cerebellum to the motor centers. 

 Impulses from the skin, the lateral line organs and the ear are 

 given directly or indirectly to the large cells of the acusticum and 

 cerebellum. The neurites from a part of these cells form bundles 

 which have been described as going down close to the ventricle, 

 to the somatic motor column (p. 135). Such bundles have been 

 seen in selachians going to the nucleus of each of the eye-muscle 

 nerves. A part of the fibers enter the fasciculus longitudinalis 

 medialis and may go to the segments of the somatic motor column 

 in the spinal cord which control the movements of the body and 

 limbs. A third and more complex course for such impulses is 

 that by way of the roof of the mesencephalon (cf. p. 117). 

 The fibers descending from the tectum form the tractus tecto- 

 bulbaris. The end branches of these fibers make direct or indirect 

 connections with the motor centers in the medulla oblongata and 

 spinal cord. Finally, when in a mammal or man such impulses 

 are carried up to the cerebral cortex and give rise to a sensation, 

 there may follow a voluntary motor impulse which descends over 

 the fibers of the pyramidal tract and reaches the motor centers 

 of the spinal cord. The simpler courses for tactile impulses 

 from the cutaneous to the somatic motor nerves are indicated in 

 Figure 59, p. 118. The student should construct similar diagrams 

 to illustrate the course of impulses from the ear and the eye. 



It is not yet certainly known whether visceral sensory impulses 

 (from the general visceral surfaces, from taste organs or the 

 olfactory organs) go directly to the somatic motor nuclei. Further 

 study of the central gustatory apparatus may be expected to throw 

 light on this question. Until such investigations are made it can 

 only be said that in general -the connections between the visceral 

 sensory and somatic motor nerves are much more indirect than 

 those between the cutaneous and somatic motor nerves. Some 

 of the indirect paths of taste impulses leading to somatic motor 

 centers are shown in Figure 92, p. 174. 



