54:0 Dynamic Theory. 



part of the cord in which they are situated. The fibres of these two 

 roots come together and form a bundle, one on each side of the spinal 

 cord. Before they join, however, the posterior fibres pass through a 

 swelling or bunch of gray vesicular matter constituting the ganglion of 

 the posterior root. After the junction of these two sets of nerves, they 



FIG. IQZ.Cerebro-Spinal Axis. ( Quain, after BOUT 

 gcry.) 



F, T, External right side of cerebrum, on the 

 frontal, temporal and occipital lobes. 



C Cerebellum. 



P. -On the Pons Varolii. Between P and V is the large 

 ganglion of the 5th nerve (trigeminum or trifacial). 



mo. Opposite and left of Medulla Oblongata. 



ins & m. s. Point to upper and lower extremities of 

 spinal cord ( spinal marrow). 



ce. On lowest lumbar vertebral spine marks the cauda 

 equina (horse tail ). The spinal cord at the bottom di- 

 vides into a bush of fibres, called by this name. 



CL First Cervical Nerve. 



CVIII. Eighth, or lowest, cervical nerve. 



DI First dorsal nerve. 



D XII. Last dorsal nerve. 



LI. First lumbar nerve. 



L V. Last lumbar nerve. 



til. First sacral nerve. 



S V Fifth sacral nerve. 



CoL Coccygeal nerve. 



/S'. Left sacral plexus. 



The backbone is split down through the middle. 



pass on out together towards the skin and 

 muscles, subdividing into numerous threads as 

 they go. It has been ascertained that the 

 fibres which leave by the posterior root are af- 

 ferent or sensory nerves, which connect with 

 the skin and convey to the spinal cord the 

 stimuli of touch. Those fibres which leave 

 by the anterior root are efferent nerves, which 

 convey the return, or motor, stimulus to the 

 muscles. A part of the afferent fibres stop in 

 the gray vesicles, which constitute a consider- 

 able part of the interior of the spinal oord, 

 and so do a part of the motor fibres, so that 

 some afferent stimuli pass into this gray mat- 

 ter by the nerves of sense, and, without stop- 

 ping, pass on out as motor stimuli directly to the muscles, which there- 

 upon contract. Actions thus simpty and directly brought on, are called 

 reflex, and involuntary. But a part of the afferent fibres, and of the 

 motor fibres as well, do not connect with the gray vesicular matter in 

 the spinal cord, but pass up the cord and do not connect with any gray 

 matter till they reach the basal ganglia, or the cortex of the brain. Ac- 

 tions resulting from stimuli passing along these fibres, are apt to be 

 modified by stimuli from other parts, perhaps delayed or counteracted. 

 Such actions are said to be either voluntary or sensori-motor. 



Thus a pair of nerves, one right and the other left, leaves the spinal 



FIG. 263. 



