476 THE ORGANS 



columns with collaterals and terminals to anterior horn cells of 

 different levels, (c) Peripheral ejferent neurones as in the two-neu- 

 rone reflex. Such a three-neurone or disynaptic reflex arc may 

 involve segments above or below the segment of entrance of the 

 stimulus and is uncrossed or crossed according as the cord neurones 

 are tautomeric or heteromeric. 



The independence of the cord as a reflex mechanism is much diminished in 

 man. 



(3) A Cerebellar Arc may be constituted as follows: (a) Peripheral 

 afferent neurones to {b) column cells in cord {e.g., Clarke's column) 

 via spino-cerebellar tracts to cerebellar cortex; (c) various associative 

 cortical cerebellar neurones; {d) axones of cortical cells to (e) dentate 

 nucleus the axones of which (superior peduncle) pass to (/) nucleus 

 ruber via rubro-spinal tract to {g) efferent peripheral neurones in 

 cord. Another arc would consist of {a), (b) and (c) the same, (d) 

 cerebellar cortex to nucleus fastigii in cerebellum to (e) nucleus of 

 Deiters to (/) efferent peripheral neurones to eft'ectors (Figs. 331 and 



345)- 



Fig. 330. — Diagram showing the Most Important Direct Paths which an Impulse 

 follows in passing from a Receptor (S) to the Cerebral Cortex and from the latter back to 

 an Effector (M) {e.g., muscle), also some of the cranial-nerve connections with the cere- 

 bral cortex. Groups of neurones are represented by one or several individual neurones. 

 A, Sensory cortex; B, motor cortex; C, level of third nerve nucleus; D, level of sixth and 

 seventh nerve nuclei ; E, level of fillet or sensory decussation ; F, level of pyramidal or motor 

 decussation; G, spinal cord. 



From Periphery to Cortex. 



Neurone No. i. — -The Peripheral afferent Neurone: i, Spinal, cell bodies in spinal 

 ganglia; receptor, S, peripheral arm of spinal ganglion cell; central arm of spinal ganghon 

 cell as fibre of dorsal root to column of GoU or of Burdach, thence to nucleus of one of 

 these columns in the medulla. Vi, Cranial (example, fifth cranial nerve, trigeminus; 

 cell bodies in Gasserian ganghon); receptor; peripheral arm of Gasserian ganglion cell; 

 central arm of Gasserian ganglion cell to medulla as afferent root of fifth nerve, thence 

 to terminal nuclei in medulla. 



Neurone No. 2. — 2, Spinal connection — Cell body in nucleus of GoU or of Burdach; 

 axone passing as fibre of fillet to thalamus. V 2, Cranial nerve connection (trigeminal), 

 cell body in one of trigeminal nuclei in medulla, axone as fibre of secondary trigeminal 

 tract to thalamus. 



Neurone No. 3. — 3, Cell body in thalamus, axone passing through internal capsule 

 to termination in cortex. (Various association neurones in cortex omitted.) 



From Cortex to Periphery 



Neurone No. 4. — 4, Cell body in motor cerebral cortex; axone through internal cap- 

 sule and pes to {a) motor nuclei of cranial nerves {h) by means of pyramidal tracts to 

 ventral gray of spinal cord. 



Neurone No. 5. — 5, Spiml, cell body in ventral gray of cord; axone as motor fibre 

 of ventral root through mixed spinal nerve to effector (muscle). 



Neurone No. 5. — Cranial — Vb, Cell body in motor nucleus of trigeminus; axone 

 passing to muscle as motor fibre of fifth nerve. 



Illh, Peripheral efferent neurone of third nerve — oculomotor. F/5, Peripheral 

 efferent neurone of sixth nerve — abducens. F//5, Peripheral efferent neurone of 

 seventh nerve — facial. Xlli, Peripheral efferent neurone of twelfth nerve — hypo- 

 glossal. 



