THE SPINAL CORD g r 



vical or first thoracic to the second or third lumbar segments. It is a prom- 

 inent feature in cross-sections of the thoracic cord, appearing as a well-defined 

 oval area richly supplied with collaterals from the dorsal roots. The cells have 

 an oval or pyriform shape; each has several dendritic processes and an axon 

 which enters the lateral funiculus, within which it runs toward the cerebellum 

 in the dorsal spinocerebellar tract. 



The Spinal Reflex Mechanism. In the next chapter we will consider at 

 length the long ascending and descending paths in the white substance of the 

 cord by which afferent impulses from the spinal nerves reach the brain, and 

 those through which the motor centers of the brain exert in return a controlling 

 inf uence over the spinal motor apparatus. But fully as important as these are 

 the purely intraspinal connections the spinal reflex mechanism. 



Fig. 66. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a 

 simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; 

 /, commissural neuron. 



A reflex arc in its simplest form may be made up of only two neurons, the 

 primary sensory and motor neurons with a synapse in the gray matter of the 

 anterior column (Fig. 66). It consists of the following parts: (1) a receptor, 

 the peripheral sensory endings; (2) a conductor, the afferent nerve-fiber; (3) a 

 center, including the synapse in the anterior column; (4) a second conductor, 

 the efferent nerve-fiber, and (5) an effector, the muscle-fiber. Usually, how- 

 ever, there are interposed between the primary sensory and motor elements 

 one or more intermediate neurons. These, when restricted to one side of the 

 cord, are known as association neurons; when their axons cross the median 

 plane, as many of them do through the anterior white commissure, they are 

 called commissural neurons. When the circuit is complete within a single neural 



