THE SPINAL CORD AND ITS NERVES 



139 



and limbs. There is a similar but anatomically distinct path- 

 way to the thalamus for cutaneous sensibility from the head, 

 which is called the trigeminal lemniscus (see p. 180 and Figs. 

 64, 77, 81). 



Within the spinal cord the nerve-fibers of sensibility to pres- 

 sure, pain, and temperature run in three distinct tracts of the 

 i 



Fasciculus graeilis "1 



Vproprioceptive 

 'Fasciculus cuneatus j 



Dorsal spino-cere- 

 bellar tract (pro- 

 prioceptive) 



Nucleus dorsalis of 

 Clarke 



Ventral spino-cere- 

 bellar tract (pro- 

 prioceptive) 



Spinal lemniscus 

 (exteroceptive for 

 pain, heat, and 

 cold) 



Spinal lemniscus 

 (exteroceptive for 

 touch and pres- 



sure) 



Fig. 63. Diagram to illustrate the terminations within the spinal cord 

 of some of the types of somatic sensory fibers and their secondary paths. 

 The central connections of root fibers 1, 2, and 5 provide for proprioceptive 

 responses; those of fibers 3 and 4, for exteroceptive responses. Root fiber 

 1 terminates in the nucleus of the fasciculus cuneatus of the same side at 

 the upper end of the spinal cord and conveys impulses of muscular sensi- 

 bility, sense of passive position and movement, and of spatial discrimina- 

 tion. Root fiber 2 terminates in the nucleus dorsalis of Clarke (Clarke's 

 column) and root fiber 5 in the same nucleus or adjacent parts of the gray 

 substance. These fibers call forth unconscious cerebellar activity underly- 

 ing the coordination and reflex tone of the muscles. Root fibers 3 and 4 

 terminate in the dorsal gray column and convey exteroceptive impulses. 

 Fiber 3 typifies all fibers which carry sensibility of pain, heat, and cold; 

 fiber 4, those which carry sensibility of touch and pressure. 



spinal lemniscus (the pain and temperature tracts very close 

 together, see Figs. 59, 63, and 81), so that it occasionally hap- 

 pens that one may be destroyed by accident or disease without 

 affecting the other two. Thus, at the level of the fifth cervical 

 vertebra the destruction of the pathway for touch and pressure 

 (tractus spino-thalamicus ventralis of Fig. 59) would result in 

 the total loss of both cutaneous and deep sensibility to pressure 



