132 



HISTOLOGY 



muscles to act voluntarily. The descending fiber crosses to the opposite 

 side during its descent, and occupies the position in the cord shown in Fig. 

 123, i. A branch is shown passing to the motor cell, d. 



From this sketch of the constitution of the nervous system, it is seen 

 that it consists essentially of cells, made up of cell bodies and of fibers; the 

 fibers are prolongations of the cell bodies. The cells are sensory, or afferent, 

 conveying impulses toward the central nervous system; and motor, or 

 efferent, conveying impulses away from the central system. Within the 

 cord these cells connect with others, forming ascending and descending 

 tracts, or bundles of fibers passing toward the brain and away from it, 

 respectively. Fibers which serve to connect different levels of the cord 

 with one another are known as association fibers; those which connect 

 the opposite sides are commissural fibers. 



Certain features in the development of the nervous system in lower animals, of 

 interest in connection with the mammalian nervous system, are shown diagram- 

 matically in Fig. 124. In sponges, according to Parker, there is no nervous tissue of 

 any sort, but beneath the thin epithelium he finds elongated contractile cells which 

 "resemble primitive smooth muscle fibers" (Fig. 124, A). They have been regarded 

 as modified epithelial cells. Parker finds that they are stimulated directly, as a result 



FIG. 124. A, DIAGRAM OF THE MUSCULAR MECHANISM IN A SPONGE. (Parker.) B, DIAGRAM OF THE 

 NBURO-MUSCULAR MECHANISM IN A MEDUSA. (Parker, after Hertwig.) C, DIAGRAM OF THE 

 VENTRAL NERVOUS CHAIN (c) AND ADJACENT STRUCTURES IN AN EARTHWORM. (Parker, after 

 Retzius.) 



Longitudinal muscle; b, motor fiber; d, sensory fiberj e, epithelium on the under surface of the 

 body, containing neuro-epithelial cells. 



of changes in the sea- water, so that they slowly contract and close the orifices around 

 which they are situated. Since the sponges are lower than any animals which are 

 known to have nerve cells, Parker concludes that muscular tissue arose independently 

 of nervous tissue, and is the more primitive (Journ. Exp. Zool., 1910, vol. 8, pp. 1-41). 



In the medusae, neuro-epithelial cells, nerve cells, and both smooth and striated 

 muscle fibers are present. According to Oskar and Richard Hertwig, the muscle 

 cells are derived from the deep part of the ectodermal epithelium, and from the 

 first they are connected with nerve cells or neuro-epithelial cells (Fig. 124, B). In 

 other words, in the medusas muscle and nerve develop in primary communication 

 with one another (Das Nervensystem der Medusen, Leipzig, 1878). 



In the earthworm (Fig. 124, C) neuro-epithelial cells in the ventral body wall 

 send fibers to a cord of nervous tissue which constitutes a central nervous system. 

 From cells in this cord, processes extend to the muscles, as shown in the diagram. 

 Thus the neuro-epithelial cell does not stimulate the muscle directly; it conveys an 

 impulse to the motor cell which in turn acts upon the muscle. In addition to the 



