764 





order from the thalamus to the cerebral cortex; (2) impulses of unconscious muscle 

 sense, via neurons of the second order from the nucleus gracilis and nucleus cuneatus 

 pass in the internal and external arcuate fibers of the medulla oblongata to the? 

 inferior peduncle and through it to the cerebellum ; (3) impulses of tactile discrimina- 

 tion, via neurons of the second order from the nucleus cuneatus and nucleus gracilis 

 pass in the median lemniscus to the thalamus, neurons of the third order pass from 

 the thalamus to the cortex. 



The Posterior Proper Fasciculus (posterior ground bundle; posterior basis bundle) 

 arises from cells in the posterior column; their axons bifurcate into ascending and 

 descending branches which occupy the ventral part of the funiculus close to the 

 gray column. They are intersegmental and run for varying distances sending off 

 collaterals and terminals to the gray matter. 



Some descending fibers occupy different parts at different levels. In the cer- 

 vical and upper thoracic regions they appear as a comma-shaped fasciculus in 

 the lateral part of the fasciculus cuneatus, the blunt end of the comma being 

 directed toward the posterior gray commissure; in the lower thoracic region they 

 form a dorsal peripheral band on the posterior surface of the funiculus; in the lumbar 

 region, they are situated by the side of the posterior median septum, and appear 

 on section as a semi-elliptical bundle, which, together with the corresponding 

 bundle of the opposite side, forms the oval area of Flechsig; while in the conus 

 medullaris they assume the form of a triangular strand in the postero-medial part 

 of the fasciculus gracilis. These descending fibers are mainly intersegmental 

 in character and derived from cells in the posterior column, but some consist 

 of the descending branches of the posterior nerve roots. The comma-shaped 

 fasciculus was supposed to belong to the second category, but against this view 

 is the fact that it does not undergo descending degeneration when the posterior 

 nerve roots are destroyed. 



Roots of the Spinal Nerves. As already stated, each spinal nerve possesses 

 two roots, an anterior and a posterior, which are attached to the surface of the 



medulla spinalis opposite the 



Lateral column corresponding column of gray 



substance (Fig. 675) ; their fibers 

 become medullated about the 

 fifth month of fetal life. 



The Anterior Nerve Root (radix 

 anterior) consists of efferent fibers, 

 which are the axons of the nerve 

 cells in the ventral part of the 

 anterior and lateral columns. A 

 short distance from their origins, 

 these axons are invested by medul- 

 lary sheaths and, passing forward, 

 emerge in two or three irregular 

 rows over an area w r hich meas- 

 ures about 3 mm. in width. 



The Posterior Root (radix pos- 

 terior) comprises some six or eight 

 fasciculi, attached in linear series 

 along the postero-lateral sulcus. 

 It consists of afferent fibers which 

 arise from the nerve cells in a spinal ganglion. Each ganglion cell gives off a single 

 fiber which divides in a T-shaped manner into two processes, medial and lateral. 

 The lateral processes extend to the sensory end-organs of the skin, muscles, tendons, 

 joints, etc. (somatic receptors), and to the sensory end-organs of the viscera (visceral 



Ant. med. fissure-^ 

 Anterior column'*' 



Posterior 

 column 



Anterior root*.-? 



Fio. 675. A spinal nerve with its anterior and posterior roots. 



