THE MAMMALIAN NERVOUS SYSTEM 63 



neurones involved in these reflexes lie in the gray matter, and 

 their axones form the greater part of the fasciculus proprius, 

 through which reflex impulses are transmitted in both ascending 

 and descending directions between the different levels of the 

 cord. With the aid of your reference books build up a clear 

 picture of the mode of connection of these neurons in typical 

 spinal reflexes. See Herrick ('15), Figs. 60, 61; Herrick and 

 Coghill ('15); Howell ('15), Chapters VII, VIII; Morris ('14), 

 Fig. 610, p. 767; Quain ('09), p. 99; Sherrington ('06), Chapters 

 I to IV, especially the diagram on p. 46; Starr, Strong, and 

 Learning ('96). 



67. Functional analysis of the cord. — (a) The classification of 

 the functional systems adopted in this work should here be re- 

 viewed (see Sections 15 to 19, 47, 52 to 57, and the references 

 there given). The somatic sensory systems include the nerves, 

 .centers, and correlation tracts of general cutaneous and deep 

 sensibility (in muscles, joints, etc.) and in the head the optic, 

 auditory, and cerebellar systems. These are all concerned with 

 the adjustment of the body to its external environment. They 

 fall into two subdivisions: (1) exteroceptive, and (2) propriocep- 

 tive (see Herrick ('15), Chapters V and IX; Sherrington ('06) 

 Chapter IV) , whose conduction pathways and correlation centers 

 are distinct. The exteroceptive systems respond to external 

 excitations; the proprioceptive systems to excitations arising 

 within the body, but subsidiary to the somatic motor reacting 

 system. The optic and cochlear systems constitute highly 

 differentiated or special members of the exteroceptive series, and 

 the vestibular system is similarly a special proprioceptive ap- 

 paratus. 



(b) Review now the functional composition of the spinal 

 nerves (see Herrick ('15), Figs. 55 and 56) and master the topo- 

 graphic relations of the internal fiber tracts related to the 

 various functional systems. (The central relations of the visceral 

 sensory components of the spinal nerves are not accurately 

 known.) The primary somatic motor and visceral motor centers 

 of the cord are distinct and are easily recognized in Nissl prepara- 

 tions. The somatic motor neurons lie in the ventral gray col- 

 umn and the visceral motor in the intermedio-lateral column (see 

 Herrick ('15), Fig. 56). 



