890 CENTRAL NERVOUS SYSTEM 



tion of fibers to the hypoglossal nerve produces a similar alternating 

 paralysis of the muscles of the tongue and of the limbs in case of lesions 

 in the medulla. The tongue is paralyzed on the same side as the lesion, 

 the arms and legs on the opposite side, while the facial muscles of 

 both sides escape. Such a condition is rare, however. Unilateral lesions 

 below the decussation in the medulla which affect the pyramidal tracts 

 cause paralysis which may affect those muscles whose motor neurons lie 

 below the level of the lesion and on the same side of the body. If the af- 

 ferent paths in the cord are affected also, sensation will be disturbed, 

 and, as we have seen, heat, cold and pain may be lost over the opposite 

 side of the body, the sense of position, passive movement, and two di- 

 mensional localization will be impaired over the same side of the body, 

 while touch may be unimpaired on both halves. 



The Peripheral Distribution of Efferent Nerves 



The motor neurons which conduct impulses for voluntary movement 

 from the central nervous system to the muscles have their cell bodies 

 in the ventral horn of the grey matter of the cord. Like the primary 

 afferent neurons they have a segmental origin, and are distributed to those 

 muscles which arose in the corresponding segments of the embryo. The 

 segmental arrangement of the muscles has become greatly obscured dur- 

 ing development, especially in the limbs, so that only in the case of 

 the intercostal muscles does it remain perfectly evident. By the careful 

 study of comparative anatomy and by correlating lesions, such as may 

 occur in anterior poliomyelitis, which affect only a single segment of 

 the grey matter, with the resulting muscular paralysis, it has been pos- 

 sible to make out the segmental origin and innervation of the various 

 muscles of the body. 



In man the distribution of the ventral root fibers according to seg- 

 ments for the cervical and lumbosacral regions is as follows: 



05 Deltoid, biceps, brachialis, supinators, rhomboids. Occasionally 



radial extensors. Barely pronator radii teres. 



06 Pronators, radial extensors, pectoralis major (clavicular fibers), 



serratus anticus. 



07 Triceps, extensor carpi ulnaris, extensors of fingers, pectoralis 



major. 



08 Flexors of wrist and fingers. 

 Tl Intrinsic muscles of hand. 



S3, 4 Levator ani, sphincter ani, perineal muscles. 



S2 Glutei, biceps, semitendinosus and semimembranosus. 



SI Intrinsic muscles of foot, tibialis posticus, and muscles of calf. 



L5 Muscles of ventrolateral leg (except tibialis anticus). 



L4 Extensors of leg and tibialis anticus. 



