SPINAL CORD. 167 



cerebellum; it first appears in the lumbar region, and increases in 

 thickness as it passes upward. 

 (c) The ventral tract, lying just posterior to the ventral cornua. 



3. A dorsal column, the portion included between the dorsal roots and the 

 dorsal fissure, also divisible into two portions: 



(a) An inner portion, the poster o-internal column, or the column of Goll, 



bordering the dorsal median fissure, and 

 (6) An external portion, the poster o-external column, the column of 



Burdach, lying just behind the dorsal roots. 



The two portions of the dorsal column are composed of long and short 

 commissural fibers, which connect different segments of the spinal cord. 



The Relation of the Spinal Nerves to the Spinal Cord. The spinal 

 nerves present near the spinal cord two divisions which from their connection 

 with the anterior or ventral and the posterior or dorsal surfaces are known 

 as the anterior or ventral and posterior or dorsal roots. The ventral roots 

 are composed of nerve-fibers which have their origin in the nerve-cells in 

 the anterior horns of the gray matter. The dorsal roots are composed of 

 nerve-fibers which have their origin in the nerve-cells in the spinal ganglia. 

 After entering the cord some of the posterior or dorsal root-fibers arborize 

 around nerve-cells in the gray matter at the same level; others pass obliquely 

 upward through the posterior white columns as far as the nucleus gracilis 

 and the nucleus cuneatus around the nerve-cells of which they terminate. 



FUNCTIONS OF THE SPINAL CORD. 



The spinal cord, by virtue of its contained nerve-cells and nerve-fibers, 

 may be regarded as composed of 



1. Independent nerve centers each of which has a special function; and 



2. Of conducting paths by which these centers are brought into relation with 

 one another and with the cerebrum and its subordinate or underlying parts. 



i. As an Independent Nerve Center. 



The spinal cord, by virtue of its contained nerve-cells, is capable of trans- 

 forming afferent nerve impulses arriving through the afferent nerves into 

 efferent impulses, which are reflected outward through efferent nerves to 

 muscles, producing motion; to glands, exciting secretion; to blood-vessels, 

 changing their caliber. All such actions taking place independent of either 

 sensation or volition are termed reflex actions. The mechanism involved in 

 every reflex action consists of a receptive surface, an afferent nerve, an 

 emissive center, an efferent nerve, and a responsive organ, muscle, gland, or 

 blood-vessels. 



