196 NERVOUS SYSTEM OF VERTEBRATES. 



mid- ventral furrow (Figs. 3, 46, etc.). The motor cells are large 

 and have large dendrites whose branches spread widely through the 

 latero- ventral part of the brain or cord. The neurites from the cells 

 which lie in the immediate vicinity of a nerve root may pass out in 

 that root, but most neurites must run forward or backward in order 

 to reach their nerve roots. Indeed, it is probable that the neurites 

 do not all enter the nerve root which lies nearest to their cells of 

 origin but that many neurites run through one or more complete 

 segments before going out of the brain or cord as fibers of a ventral 

 nerve. The neurites which run from segment to segment lie 

 mesial to the motor nuclei, where they run in a definite bundle 

 at either side of the mid-ventral groove of the ventricle. The 

 large bundle of coarse fibers in this position is known as the fasci- 

 culus longitudinalis mediates (dorsalis or posterior), and forms 

 one of the most conspicuous landmarks in the brain of any verte- 

 brate. This contains, however, other fibers in addition to those 

 here mentioned. That portion of it which is made up of motor 

 fibers on their way to become fibers of ventral nerves may be 

 called the somatic motor fasciculus. The motor cells together 

 with this fasciculus make up the somatic motor column or zone 

 of the spinal cord and brain. In those segments of the brain which 

 have no ventral nerves the somatic motor cells are wanting and 

 the column is represented only by the somatic motor fasciculus. 

 This fasciculus continues forward beyond the oculomotor nerve 

 and its fibers take their origin from the cells of a special nucleus 

 cephalad from the nucleus of the III nerve in the ventral part 

 of the central gray of the thalamus. This nucleus is frequently 

 considered to have some relation to the fibers of the posterior 

 commissure (cf. p. 265 and Fig. 134, p. 272). 



Owing to the importance of bodily movement in nearly all the 

 activities of the animal, the connections of the somatic motor 

 centers with the rest of the nervous system are numerous and varied 

 in character. Necessarily the motor neurones form a link in 

 every reflex chain which leads to a bodily movement, whatever 

 the source and character of the exciting stimulus. Some of the 

 chief classes of impulses and the tracts which bring them to the 

 motor centers will be mentioned here. The simplest case is that 



