316 Basic Structure of Vertebrates 



of the fiber, the end of the nerve often showing a knotlike 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 (Figs. 255, 256) 

 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 sarco- 

 lemma is less strongly developed than in somatic fibers. A peculiar 

 feature of the cardiac fiber is the presence of conspicuous transverse 

 bands, the intercalated disks (Fig. 257), which are quite distinct 

 from the ordinary striations. 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 is 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 physiologic continuity with the nucleated region of the 

 cell of which it is an integral part. Any cell engaged in nervous oper- 

 ations, 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 

 physiologically 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. 258, 259). Unipolar 

 cells, of comparatively rare 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 arbori- 

 zation. The apparently "unipolar" cell of a spinal ganglion is a dis- 

 guised bipolar cell (Fig. 260). 



Most types of receptor neurons are epithelial. In some of these the 



