78 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



The Muscle Spindles. Voluntary muscles are supplied with nerve 

 terminations of a sensory nature ending in Pacinian corpuscles, in end bulbs, 

 and in special structures known as neuromuscular bundles or muscle spindles. 

 A muscle spindle consists of one or more muscle fibers somewhat smaller than 

 the typical fibers of that particular muscle, and containing a relatively great 

 amount of sarcoplasm, and many nuclei. These fibers are intimately bound 

 with nerve terminations as shown in the figure 106. Certain of the vol- 

 untary muscles, particularly those of the arms and legs, contain large num- 



S 



FIG. 106. A section of a muscle spindle from a. voluntary muscle of the cat. 

 A, Annular terminations; S, spiral terminations; F, arborescent terminations. (From 

 Barker, after Ruffini.) 



bers of muscle spindles. In the tendons of these muscles also are numerous 

 muscle-tendon organs of the Golgi type. Sherrington conclusively demon- 

 strated the sensory nature of these fibers and terminations by showing that 

 they did not undergo Wallerian degeneration when the corresponding anterior 

 spinal nerve roots were cut and allowed to degenerate. 



THE NEUROGLIA. 



The neuroglia, while not a nervous tissue, is closely mingled with it and 

 forms an important constituent of the nervous system. It consists of cells 

 giving off a fine network of richly branching fibers. Neuroglia is a form 

 of connective tissue, and it is in its functions strictly comparable to the con- 

 nective tissue which supports the special structures of other organs, like the 

 lungs and kidneys. In the adult animal the neuroglia tissue is composed of 

 cells from which are given off immense numbers of fine processes. These 

 extend out in every direction, and intertwine among the nerve fibers and nerve 

 cells, figure 105. The neuroglia cell differs in size and shape very much in 

 different parts of the nervous system in accordance with the arrangement of 

 the nervous structures about it. The cell is composed of granular proto- 

 plasm, and lying in it is a large nucleus, within which is a nucleolus. The 

 body of the cell is small in proportion to the nucleus. 



