96 CHORDATE ANATOMY 



The relation of an unstriated fiber to its nerve is apparently of the 

 simplest sort. A terminal twig of nerve merely attaches to the surface 

 of the fiber, the end of the nerve often showing a knot-like enlargement. 

 Presumably every striated fiber has a nerve connected to it. The nerve, 

 however, enters a small flat plate of nucleated protoplasm lying super- 

 ficially on the muscle fiber. Within this motor plate (Fig. 89) the nerve 

 ramifies into fine twigs which seem to terminate in the substance of the 

 plate. 



Striated fibers are bound together in bundles enwrapped by a con- 

 nective-tissue perimysium. Thick muscles consist of several or many 

 such bundles wrapped together. 



Cardiac muscle has striations which resemble those of somatic muscle 

 but the fibers are relatively short and are branched. The sarcolemma is 

 less strongly developed than in somatic fibers. A peculiar feature of the 

 cardiac fiber is the presence of conspicuous transverse bands, the inter- 

 calated discs (Fig. 90) which are quite distinct from the ordinary stria- 

 tions. Their significance is not known. 



Nervous Tissue 



All nervous functions are carried on by protoplasm organized, as 

 always, in cells. To say, as is often done, that nervous tissues consist 

 of nerve cells and nerve fibers is inaccurate. So far as known, every 

 fiber which conducts nervous impulses is developed as an outgrowth 

 from a cell and can function and survive only so long as it remains in 

 physical and physiological continuity with the nucleated region of the 

 cell of which it is an integral part. Any cell engaged in nervous operations, 

 together with all conducting fibers which have grown out from it, is called 

 a neuron. 



A central nervous organ is a more or less complex system of physiologi- 

 cally related neurons serving for the proper association, coordination and 

 integration of nervous impulses. A ganglion is a minor localized nerve 

 center consisting of the cell-bodies of neurons together with the adjacent 

 regions of their nerve processes. 



Neurons are of various types depending on the form of the cell-body 

 and the number of nerve processes (Figs. 91, 92). Unipolar cells, of 

 comparatively rate occurrence, have a single process; bipolar neurons 

 are usually spindle-shaped and have a process at each end; multipolar 

 cells have several processes of which one, the neuraxon (axon or neurite), 

 is relatively long, while the short dendrites branch out into fine twigs 

 which end within a short distance of the cell-body. The neuraxon may 

 give off lateral branches (collaterals) and its distal extremity breaks up 

 into fine branches forming the terminal arborization. 



