GENERAL ZOOLOGY 



Myelin sheath 



Fig. 3.18. Nervous tissue. A, typical bipolar neuron from the olfactory epithelium. B, 

 transformation of a bipolar neuron into the type found in the dorsal root ganglia of spinal 

 nerves. C, multipolar neuron, showing cytosome with numerous dendrites and a single axon. 

 D, bundle of myelinated nerve fibers surrounded by fibrous connective tissue, as in the spinal 

 and cranial nerves; each nerve fiber is surrounded by a myelin sheath. E, portion of a single 

 myelinated nerve fiber; the interruption in the myelin sheath is called a node of Ranvier; a 

 nucleus is seen in the neurilemma, or outer membrane. F, portion of an unmyelinated nerve 

 fiber, characteristic of autonomic nerves; a nucleus of the neurilemma is shown. 



These units branch and unite in such a way that a network is formed. The 

 fibrillae of cardiac muscle are made up of regions of different density so that 

 the cytoplasm presents an irregularly striated appearance. 



Striated muscle is known sometimes as skeletal muscle because it is attached 

 to the bones and by its contractions produces motion of body parts which are 

 supported by bones (Fig. 3.17Z) and E). Since these muscles can be co- 

 ordinated consciously, they are also called voluntary muscles. They appear 

 striated because the fibrillae have regions of different density, which occur at 

 such regular intervals as to give a distinct cross-striped appearance to the 

 cytoplasm. The cells are cylindrical, sometimes very long, and each contains 

 many nuclei; that is, the cells are multinucleate. Fibrous connective tissue 

 serves to bind together striated muscle cells and forms sheaths that enclose 

 great numbers of these cells which make up the visible muscles, such as the 

 gastrocnemius or the biceps (Fig. 3.17C). These connective tissue sheaths 

 are continuous with the tendons by means of which muscles are attached 

 to bones. 



Nervous Tissue. The cells of nervous tissue are differentiated in such a 

 way that they are capable of receiving stimuli in some regions, of conducting 

 nerve impulses from one part of the body to another, and of discharging 

 these impulses. The general functions of nervous tissue can, therefore, be 

 stated as reception, conduction, and discharge. These activities make pos- 

 sible the coordination of the organism as a whole and will be discussed in the 

 next chapter. A nerve cell, or neuron, is composed of a nucleus surrounded by 

 a relatively small cytosome, which is prolonged into two or more processes of 

 varying lengths; these nerve fibers are of two types. Some taper along their 



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