BASIC DEVELOPMENTAL FEATURES 809 



reflex mechanism is an arrangement of neurons known as the reflex arc. Theo- 

 retically, a simple type of reflex arc would possess (fig. 352A): 



( 1 ) a sense receiving structure, the receptor; 



(2) the sensory neuron, whose long afferent or sensory fiber contacts the 

 sensory receptor, while its efferent fiber or axon continues from the 

 body of the neuron to the central nervous system. Within the central 

 nervous system the terminal fibers (telodendria) of the eff"erent fiber 

 of the sensory neuron forms a synapse with 



(3) the dendrites of an efferent neuron. From the efferent or motor neuron 

 a motor fiber (axon) leaves the central nervous system and con- 

 tinues to 



(4) the effector organ. 



Functionally, however, even the simplest type of reflex arc may not be as 

 elementary as this. More probably, a system of one or more association neu- 

 rons placed between the sensory and motor neurons exists in most instances. 



c. Structural Divisions of the Vertebrate Nervous System 



The nervous system of vertebrate animals consists of 



( 1 ) the central nervous system, a tubular structure composed of a coordi- 

 nated assembly of association neurons and their processes. The central 

 nervous system is integrated with 



(2) the peripheral nervous system constructed of a series of sensory and 

 motor neurons which connect the central nervous system with distal 

 parts of the body. Through the medium of various types of sense re- 

 ceptors the central nervous system is made aware of changes in the 

 external and internal environment of the body. 



d. The Supporting Tissue 



In addition to the irritable cellular neurons, the nervous system contains 

 connective or supporting tissue. However, unlike most of the other organ 

 systems of the body, the supporting tissue of the nervous system is derived 

 mainly from an ectodermal source. Small amounts of connective tissue of 

 mesodermal origin parallel the various blood capillaries which ramify through 

 nervous tissue, but the chief supporting tissue of the brain and spinal cord is 

 the neuroglia of ectodermal origin. The neuroglia consists of two main cellular 

 types, the ependymal cells and the cells of the neuroglia proper. 



The ependymal cells (fig. 353A) form a single layer of columnar epi- 

 thelium which lines the lumen of the neural tube. From the inner aspect or 

 base of each ependymal cell a process extends peripherad toward the external 

 surface of the neural tube (fig. 353F-H). Later the peripheral process may 

 be lost. During the earlier stages of their development the ependymal cells 

 are ciliated on the aspect facing the neurocoel (fig. 353A). 



