NERVE TISSUE. 9 1 



the cells of which they are a part are the motor cells. The efferent fibers, 

 or those which bear impulses from the central tract to the periphery, in- 

 clude the motor fibers, and also some which pass to the epithelium of glands 

 to control their activity. Besides the afferent sensory and the efferent 

 motor fibers there is a third set of commissural cells and fibers, serving to 

 connect the other two. Sensory and motor cells may connect without 

 the intervention of commissural cells, thus providing a path for the simplest 

 form of unconscious reflex action, but often one or more commissural 

 cells are interposed and the brain consists essentially of these- cells. As 

 the nervous impulse is transferred from cell to cell, being further removed 

 from the primary stimulus, it is suggested that it becomes "more sub- 

 jective and personal." 



The nervous system, then, is a median longitudinal tract of ectodermal 

 cells, divisible into afferent (sensory), efferent (motor), and commissural 

 cells. The sensory and motor cells send out processes or fibers, which 

 in bundles called nerves extend through the mesenchymal tissue to all 

 parts of the body. The central tract is called the central nervous system 

 and consists of the brain and spinal cord. The nerves constitute the 

 peripheral nervous system. Associated with the nerves there are clumps 

 of nucleated bodies of nerve cells, known as ganglia. The afferent and 

 efferent fibers to the viscera and blood vessels, together with numerous 

 ganglia, constitute the sympathetic nervous system. The nervous system, 

 therefore, is composed of central, peripheral, and sympathetic portions. 



DEVELOPMENT OF NERVE TISSUE. 



THE CENTRAL TRACT. The ectoderm in an early stage forms a flat 

 layer covering the embryo (Fig. 109 A). Along the axial line and extend- 

 ing on either side of it, the ectoderm thickens to form the medullary plate. 

 The plate becomes depressed so as to make a longitudinal groove, the 

 medullary groove [or neural groove] (Fig. 109 B). The dorsal edges of 

 the groove come together and fuse, transforming it into the medullary [or 

 neural] tube (Fig. 109 C). Thus the tube becomes separated from the 

 general layer of ectoderm which is to form the epidermis. This medullary 

 tube is the central nervous system. In its anterior part the cavity is trans- 

 formed into a series of connected dilated spaces or ventricles, and its 

 walls become very thick, thus forming the brain. The posterior part 

 makes the spinal cord; its walls are less extensively but more uniformly 

 thickened than those of the brain, and its cavity remains small, becoming 

 the central canal. This canal is continuous with the ventricles of the 

 brain and a line of division between the spinal cord and brain must be 



