926 THE NERVOUS SYSTEM 



peripheral nervous system. This system, abundantly attached to the central 

 system, consists of numerous bundles of nerve-fibres which divide and ramify 

 throughout the bodj^, anastomosing with each other and forming various plexuses, 

 large and small. The terminal rami divide and subdivide until the divisions attain 

 the individual nerve-fibres of which they are composed, and finally the nerve- 

 fibres themselves divide and terminate in relations with their allotted peripheral 

 elements. It is by means of this system that stimuli arising in the peripheral 

 tissues are conveyed to the central system, and that impulses in response are borne 

 from the central system to the peripheral organs. For purposes of description, as 

 well as upon the basis of certain differences in structme, arrangement, and dis- 

 tribution, the peripheral nervous system is separated into two main divisions: — 

 (1) the cranio-spinal and (2) the sympathetic system. 



Both of these divisions include numerous ganglia or peripheral groups of nerve- 

 cells from which arise a considerable proportion of the fibres forming their nerve- 

 trunks, but neither of the divisions may be considered wholly apart from the 

 central system nor are they independent or separate from each other. The sen- 

 sory or afferent fibres of the cranio-spinal nerves pass by way of the afferent nerve- 

 roots into the central system and contribute appreciably to its bulk, and the motor 

 or efferent fibres of these nerves have their cells of origin (nuclei) situated within 

 the confines of the central system. The sympathetic system is intimately asso- 

 ciated with the cranio-spinal, and consequently with the central system — (1) 

 by means of fibres which enter and terminate in the cranio-spinal gangha and 

 transfer impulses which enter the central system ; (2) by efferent fibres of central 

 origin which course in the nerve-trunks and terminate in the ganglia of the sym- 

 pathetic system; (3) also, the sympathetic trunks usually contain numerous 

 afferent cranio-spinal fibres which thus course to their peripheral termination, 

 usually in the so-called 'splanchnic area/ or domain of the sympathetic, in 

 company with the sympathetic fibres. Likewise the peripheral branches of 

 the cranio-spinal nerves often carry for varying distances numerous sympathetic 

 fibres which are on their way to terminate either in other sympathetic ganglia or 

 upon their allotted peripheral tissue-elements. 



The following differences between the cranio-spinal and sympathetic systems of nerves may 

 be cited: — (1) The cranio-spinal nerves are anatomically continuous with the brain and spinal- 

 cord; probably no fibres arising in the sympathetic gangha actually enter the central system 

 other than for the innervation of its blood-vessels. (2) The ganglia of the cranio-spinal 

 nerves all he quite near the central axis, in line on either side of it, and at more or less regular 

 intervals; the sympathetic gangha are scattered throughout the body tissues, are far more 

 numerous and more variable in size, and probably only the larger of them are symmetrical 

 for the two sides of the body. (3) The cranio-spinal nerves are paired throughout, and the 

 nerves of each pair are symmetrical as to their origin and also, with certain exceptions (notably 

 the vagus), in their course and distribution; most of the larger and more proximal of the sym- 

 pathetic nerve-trunks are symmetrical for the two sides of the body; many of them are not, 

 and many of the smaller and most of the more peripheral nerves and gangha, large and small, 

 are not paired at all. (4) Even in their finer twigs, the cranio-spinal nerves of the two sides 

 probably do not anastomose with each other across the mid-hne of the body; the sympathetic 

 nerves do so abundantly, especially within the body cavity. (5) The cranio-spinal nerves are 

 distributed to the ordinary sensory surfaces of the body and the organs of special sense and to 

 the somatic, striated or 'voluntary' muscles of the body; the sympathetic fibres are devoted 

 chiefly to the supply of the so-called involuntary muscles of the body, including the smooth 

 muscle in the walls of the viscera and in the walls of the blood and lymph vascular-systems, 

 while others serve as secretory fibres to the glands. (6) Cranio-spinal nerve-fibres are char- 

 acterized in general by well-developed medullary sheaths, making the nerves appear as white 

 strands; most of the sympathetic fibres are non-medullated, some are completely and some 

 partially meduUated, but none possess as thick medullary sheaths as those of the cranio-spinal 

 nerves. Thus sympathetic nerves appear as grey strands. 



The cranio-spinal nerves. — There are forty-six pairs of cranio-spinal nerves, 

 of which thirty-one pairs are attached to the spinal cord (spinal nerves) and fifteen 

 pairs to the encephalon (cranial nerves). The spinal nerves are the more primi- 

 tive and retain the typical character, i. e., each is attached to the spinal cord by two 

 roots, a dorsal or sensory ganglionated root, and a ventral, which is motor, and 

 thus not gangHonated. Most of the cranial nerves have only one root, which in 

 come cases corresponds to a dorsal root and therefore has a gangUon, and in other 

 cases corresponds, physiologically at least, to a ventral root of a spinal nerve. 

 Among other differences, the fibres of the first cranial nerve, for example, do not 

 collect to form a distinct nerve-trunk. 



Jibii 



