140 



Basic Structure of Vertebrates 



I. Nervous System 



The protoplasmic properties on which the action of nerves depends 

 are irritability and conduction. If one of several pseudopodia of 

 an ameba is touched, all of the pseudopodia may contract. Both 

 irritability and conduction are concerned in this reaction. All cells 

 of the more complex animals probably possess these two facilities, but 

 they become the special and emphasized functions of cells of the 

 nervous organs. 



Every nerve-fiber develops as an outgrowth from a nerve-cell and 

 is permanently in vital connection with the main body of the cell. 

 The structural element of the nervous system is a nerve-cell together 

 with the nerve-fiber or fibers which it has produced (Fig. 131). This 

 element is called a neuron. A cell or fiber which is specialized for 

 reception of a stimulus from any source external to the nervous system 

 is known as a sensory cell or fiber, or a receptor. If it is so situated as 

 to receive stimuli from sources external to the animal, it is called an 

 exteroceptor. A receptor situated on some internal surface — e.g., the 

 lining of the stomach — and thus exposed to stimulation by some agency 

 in the adjacent cavity, is called an interoceptor. Proprioceptors 

 are receptors which are buried within masses of tissues — e.g., a muscle, 

 a tendon, or at a joint. Nerves are conductors (Fig. 132). A central 

 nervous organ is a more or less complex system of physiologically 

 connected neurons which effect the coordination and integration of 

 nervous impulses. A ganglion is a minor center consisting of cell- 

 bodies of neurons together with the adjacent region of their nerve 

 processes. A conductor which transmits impulses toward a central 



Fig. 131. Types of nerve-cells. (A) Multipolar cell. (B) Portion of nerve-fiber 

 with sheaths. (C) Unipolar cell (such a cell may arise by modification of a bipolar 

 cell, as shown in Fig. 260). (D) Pyramidal cell (from cerebral cortex), (a) Axon; 

 (c) collateral; (c6) cell-body; (d) dendrites; (m) medullary sheath; (n) nucleus of 

 cell of Schwann's sheath; (r) node of Ranvier; (s) sheath of Schwann; (0 teloden- 

 dron. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, 

 The Blakiston Company.) 



