SYMPATHETIC SYSTEM. 753 



thoracic or first lumbar nerve. The last nerve in the sciatic cord may be the second, third, 

 or fourth sacral nerve. The. position of the n. furcalis is a guide to the arrangement of the 

 plexus. It may be formed by the third, third and fourth, fourth, fourth and fifth, or fifth 

 lumbar nerves. The resulting variations are illustrated by the following extreme cases : 



(1) Prefixed Variety. (2) Postfixed Variety. 

 Nervus furcalis . . . L. 3 and 4 (double). L. 5. 



Obturator . . . . L. 1, 2, 3. L. 2, 3, 4, 5. 



Femoral . . . . T. 12, L. 1, 2, 3, 4. L. 2, 3, 4, 5. 



Tibial L. 3, 4, 5, S. 1, 2. L. 5, S. 1, 2, 3, 4. 



Common peroneal . . L. 3, 4, 5, S. 1. L. 5, S. 1, 2, 3. 



Those variations in the constitution of the lumbo-sacral plexus are most numerous which 

 are due to the inclusion of nerves more caudally placed. Thus, out of twenty -two variations 

 in the position of the n. furcalis, in nineteen Eisler found it formed by the fifth lumbar nerve ; 

 in two cases only, by the third lumbar nerve. There is further evidence that variations in the 

 position of the plexus are accompanied by variations in the vertebral column itself. Out of the 

 twenty -two abnormal plexuses examined by Eisler, sixteen were coincident with abnormal 

 arrangement of the associated vertebrae. 



SIGNIFICANCE OF THE LIMB-PLEXUSES. 



From the above considerations, it is obvious that something more than convenience of transit 

 for the spinal nerves to skin and muscles is secured by the formation of the limb -plexuses. It 

 has been shown that by their combinations in the plexuses, every spot or area of skin in the 

 limbs is innervated by more than one spinal nerve ; and generally, also, each limb-muscle is 

 supplied by more than one spinal nerve. Each cutaneous area and each limb -muscle is thus 

 brought into relationship with a wider area of the spinal medulla than would occur if the 

 plexuses were non-existent. A simultaneous record of sensation is thus transmitted from any 

 given point on the surface of the limb through more than one posterior root ; and a more ready 

 co-ordination of muscular movement is brought about by the transmission of motor impulses 

 from the anterior root of a given spinal nerve to more than one muscle at the same time. In a 

 word, a plexus exists to supply the whole limb and the limb as a whole, as an organ which has 

 its different active parts connected with the central nervous system by means of the limb-plexus. 



SYSTEMA NERVORUM SYMPATHICUM. 



The sympathetic nervous system comprises a pair of elongated gangliated 

 trunks, extending through the whole length of the body from the base of the skull 

 to the coccyx, connected to the peripheral spinal nerves by one series of nerves, 

 and to the viscera by another series. At its cephalic end each sympathetic trunk 

 passes into the cranial cavity along with the internal carotid artery, on which it 

 forms plexuses, and thereby forms complex relations with certain cerebral nerves. 

 At their caudal ends the two sympathetic trunks are joined together by fine 

 filaments and unite with the coccygeal ganglion (g. impar). 



The sympathetic system is essentially dependent on and subservient to the spinal 

 nervous system. It distributes efferent fibres from the peripheral spinal nerves to 

 (a) the viscera and vessels of the splanchnic area, and (&) through recurrent (gray) 

 rami to vessels, glands, and involuntary muscles in the course of the somatic 

 divisions of the spinal nerves. It further collects and transmits to the cerebro- 

 spinal system afferent fibres from the viscera (Fig. 635). 



General Structure of the Sympathetic System. The sympathetic system is 

 composed of two elements ganglia and nerve fibres. 



Ganglia Trunci Sympathici. The ganglia are variable in number, form, size, 

 and position. They are not definitely segmental in position, but they are always 

 connected together by a system of narrow cords of nerve fibres. A ganglion 

 (Fig. 635) consists of a larger or smaller number of multipolar nerve cells, 

 enclosed in a capsule of connective tissue. Each cell is provided with one axon and 

 a number of dendrites. The axon may enter into the composition of (a) the con- 

 necting cord ; (6) a central branch (gray ramus communicans) ; or (c) a peripheral 

 branch from the sympathetic trunk. These axons are commonly medullated at 

 their origin, but become non-medullated in their course from the parent cell. 

 Besides these ganglia, two other series of ganglia are present in connexion with 

 the peripheral branches of the sympathetic : ganglia plexuum sympatMcorum, 



49 



