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Basic Structure of Vertebrates 



SPINAL CORD 



A. DIAGRAM OF A REFLEX ARC 



Fig. 133. Diagram of a nervous arc. (A) Three neurons — afferent, intercalary, 

 and efferent — are shown in their relations to one another and to the skin and 

 muscle. The intercalary neuron is located in the gray matter of the spinal cord. 

 (B) An enlarged section of a nerve-fiber. (Courtesy, Neal and Band: "Chordate 

 A.natomy," Philadelphia, The Blakiston Company.) 



mated so that the nervous impulse passes from one fibril to another 

 in a manner analogous to that in which an electric current passes from 

 one to another of two wires which are merely in contact, but not con- 

 tinuous, or even "sparks" across a small gap. This type of relation 

 between fibrils of two neurons is known as a synapse (Fig. 133 ; cells 

 in spinal cord). There is accumulating evidence that transmission 

 across a synapse is effected by instantaneous secreting of a specific 

 chemical substance by the fibrils of the neuron which is sending the 

 impulse across, this secreted "neurohumor" thus acting chemically 

 on the adjacent fibrils of the other neuron. 



Central Nervous Organs 



Some invertebrates, such as arthropods and annelid worms, have 

 a dorsal cephalic ganglion or "brain," but the central nervous organ 

 in the body is a cord, or pair of cords, situated ventrally to the diges- 

 tive tube. In flatworms there are one or more pairs of nerve-cords 

 placed more or less laterally. In no invertebrate do the central nervous 

 organs resemble closely those of vertebrates. 



Of all vertebrate organs, the central nervous organs, brain and 

 spinal cords are most constant in manner of embryonic development, 

 in position, and in general features of structure. Along the entire length 

 of the middorsal line of the early embryo, a band of the outer layer 

 (ectoderm, which otherwise gives rise to the outer layer, epidermis, of 

 the skin) becomes infolded, detached from the outer ectoderm, and 



