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THE NERVE SYSTEM 



considerably in form and size; the largest are found on the sensor root of the 

 trigeminus and in the cavity of the abdomen; the smallest, not visible to the naked 

 eye, exist in considerable numbers upon the nerves distributed to the different 

 viscera. The ganglia are invested by a smooth and firm, closely adhering mem- 

 branous envelope, consisting of dense areolar tissue; this sheath is continuous 

 with the perineurium of the nerves, and sends numerous processes into the 

 interior of the ganglion, which support the bloodvessels supplying its substance. 

 Origin and Termination of Nerves. To the central and the peripheral ending 

 of a nerve are usually given the names of "origin" and "termination." These 

 designations have been rendered inappropriate, in many cases, by the newer 

 concept of neuronic arrangement. They have not yet become obsolete, however, 

 particularly in dissecting-room anatomy, and warrant description here with a 

 certain degree of reserve alluded to above. 



A 



B 



FIG. 589. Diagrams of motor nerve endings in A. Striated muscle. B. Cardiac muscle. C. Nonstriated 

 muscle, a. Axone. t. Telodendria. (After Huber, Bohm and Davidoff, and others.) 



^ 



Origin. The origin in some cases is single that is to say, the whole nerve 

 emerges from the nerve centre by a single root; in other instances the nerve arises 

 by two or more roots, which come off from different parts of the nerve centre, 

 sometimes widely apart from each other; and it often happens, when a nerve 

 arises in this way by two roots, that the functions of these two roots are different; 

 as, for example, in the spinal nerves, each of which arises by two roots, the ventral 

 of which is motor and the dorsal sensor. The point where the nerve root or roots 

 emerge from the nerve centre is named the superficial or apparent origin, but the 

 axones of which the nerve consists can be traced for a certain distance into the 

 nerve centre to some portion of the gray substance, which constitutes the deep 

 or real origin of the nerve. The manner in which these fibres arise at their deep 

 origin varies with their functions. The centrifugal or efferent nerve fibres 

 originate in the nerve cells of the gray substance, the axones of these cells being 

 prolonged to form the fibres. In the case of the centripetal or afferent nerves 

 the axones grow inward either from nerve cells in the organs of special sense 

 (e. g., the retina) or from nerve cells in the ganglia. Having entered the nerve 



