THE SPINAL CORD 537 



In the ventral horn two well-marked groups of cells are found, one 

 situated at the ventral and inner angle, known as the ventro-median group, 

 the other situated at the ventral and lateral angle and known as the ventro- 

 lateral group. In the lower cervical and thoracic regions, in the region 

 of the lateral horn, another group of cells is found, known as the inter- 

 mediate group. In the central portion of the horn there is also a well- 

 defined group. 



The cells of the ventral horns are of large size, nucleated and multi- 

 polar. They are the modified descendants of pear-shaped cells, the 

 neuroblasts, which migrated from the medullary tube (see page 97). In 

 the course of their migration they developed dendrites which form an 

 intricate felt- work throughout the ventral horn. -One of the processes, 

 the axon, approached the surface of the cord, penetrated it, grew out- 

 ward, became covered with myelin and neurilemma, and developed into a 

 ventral root fiber. These nerve-cells, with their dendrites, axons, and 

 terminal branches, form efferent neurons of the first order. The inti- 

 mate histologic and physiologic relationship existing between the nerve- 

 cell and the axon is revealed by the degenerative changes which arise 

 in the latter when separated from the former. The cell apparently deter- 

 mines nutrition of the axon and may be regarded as trophic in function. 



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

 divided into three main groups: viz., intrinsic, receptive or afferent, and 

 emissive or efferent. 



The Intrinsic Cells. These cells are widely distributed throughout the 

 gray matter. The axons to which these cells give origin pass more or less 

 horizontally into the white matter, where they divide into branches, 

 one of which passes upward, the other downward. At various levels 

 they reenter the gray matter and arborize around other intrinsic cells. 



The Receptive Cells. These cells constitute in large part the group 

 of cells known as Clarke's vesicular column and the groups of cells situated 

 at or toward the base of the dorsal horn and intermediate gray matter. 

 They have received the names associated with them from the fact that 

 they receive nerve impulses transmitted from peripheral organs by 

 afferent nerves. The nerve impulses received by cells in Clarke's vesicular 

 column are transmitted to the cerebellum. The nerve impulses received 

 by the cells at the base of the dorsal column are transmitted by the axons 

 of the cells to the cortex of the cerebrum, where they evoke 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, thermal, tactile, etc., so these nerve-cells may be similarly classified 

 according as they transmit their excitations to those specialized areas in 

 the cerebral cortex in which these different sensations arise. 



The Emissive Cells. These cells compose the groups situated in 

 the ventral horn. The axons to which they give origin pass more or less 

 horizontally forward to the ventral surface where they emerge and become 

 known as the ventral roots of the spinal nerves. As the efferent fibers 

 in the ventral roots of the spinal nerves are classified in accordance with 

 their distribution, viz. : to skeletal muscles, glands, vascular and visceral 

 muscles and hence (see page 99) in accordance with their physiologic 

 action into motor, secretor, vaso-motor, viscero-motor and pilo-motor 



