THE SPINAL CORD. 447 



and on its inner side there is a well-marked group of cells which ex- 

 tends from the seventh or eighth cervical nerves downward to the 

 second or third lumbar nerves, being most prominent in the thoracic 

 region. This column is known as Clarke's vesicular column. From 

 the nerve- cells constituting this column axons pass obliquely outward 

 into that portion of the white matter known as the direct cerebellar 

 tract. 



Classification of Nerve-cells. The cells of the gray matter 

 may be divided into two main groups: viz., efferent and afferent. 



The efferent cells, independently of their trophic influence, are 

 also motor in function, inasmuch as the excitation arising in them 

 is transmitted outwardly through their axons to muscles, blood- 

 vessels, glands and viscera, imparting to them motion, either molar or 

 molecular. As the effaeotj&bers in the ventral roots of the spinal 

 nerves are classified (sea page 113) in accordance with their physio- 

 logic action into motor, f asp-motor, secretor, inhibitor and accelerator 

 nerves, so the nerve- cells of which the nerves are integral parts may 

 be classified physiologically as motor, vaso-motor, secretor, inhibitor, 

 and accelerator. Collections or groups of such cells are termed 

 "centers." 



The afferent cells are largely sentient or receptive in function, 

 inasmuch as the excitations brought to the spinal cord by the afferent 

 nerves in the dorsal roots from the general periphery are received 

 by them and transmitted through their axons toward the cortex of the 

 cerebrum, where they are translated into conscious sensations. As 

 the nerve-fibers in the dorsal roots of the spinal nerves are classified, 

 in accordance with the sensations to which they give rise, as sensor, 

 thejmal, tactile, etc., so these nerve-cells may be similarly classified 

 according asThey transmit their excitations to those specialized areas 

 in the cerebral cortex in which these different sensations arise. 



Structure of the White Matter. A transverse section of the 

 cord shows that the white matter completely covers the gray matter 

 except where the posterior horns reach the surface. Anteriorly the 

 Wffite matter jaf each lateral half is connected by a narrow strip or 

 bridge of white matter, the anterior comnlissure. Microscopic 

 examination shows that the white matter is composed of vertically 

 disposed medullated nerve-fibers which are devoid of a neurilemma. 

 These fibers are supported partly by a framework of connective 

 tissue, and partly by neuroglia. The white matter of each side of 

 the cord is anatomically divided into an anterior, a lateral, and a 

 posterior column by the anterior and posterior roots of the spinal 

 nerves. 



Classification of the Nerve-fibers. From a study of the 

 embryologic development of the white matter and of the degenerative 

 changes which follow its pathologic and experimental destruction, it 



