632 THE NERVOUS SYSTEM. 
The lumbo-sacral plexus shows a very considerable variability in position and constitution. 
Eisler records concomitant variations in the plexus in 18 per cent of the cases examined by 
him. The variations occur within wide limits. The plexus may begin at the eleventh or 
twelfth thoracic or first lumbar nerve. The last nerve in the great sciatic cord may be the 
second, third, or fourth sacral nerve. The position of the n. fure alis is a guide to the arrange- 
ment 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 Varvety. (2) Postfived Variety. 
Nervus furcalis . ; ; L. 3 and 4 (double) 115: 
Obturator : : : IGRI 2 Ch 1b Be At 
Anterior crural. : ; AG SPA Ibe we %, owe 1 Dig Doses IE. 
Tibial : ; : ; Lis; 4,5 Osos lene ID RRS NS) US AB ak, 
Peroneal . : ; ; Ie ek aS i 1B top Sy lly 33. 
Those variations in the constitution of the Iumbo-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 Kisler 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 vertebree. 
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 cord 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 mb through more than one dorsal root ; and a more re eady 
co-ordination of muscular movement is brought about by the transmission of motor impulses 
from the ventral root of a given spinal nerve to more than one muscle at the same time. Ina 
word, a plexus exists to sup ply 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. 
THE CRANIAL NERVES. 
There are twelve pairs of cranial nerves. They present striking differences from 
one another—in origin, in distribution, and in functions, as will be seen in the 
following table :— 
N umber. Name. | Function. pheamrione Cae 
| I. Olfactory . . | smell ‘ : f ; ; . Olfactory bulb. | 
| IT. | Optic asides : Optic thalamus. 
| Ill. Oculo-motor . Motor to the rauiccles of eyeball andl Crus cerebri. 
orbit 
TV. | Trochiear : . Motor to superior oblique muscle of | Superior medullary 
| eyeball velum, 
| V. | Trigeminal . Sensory to face, tongue, and teeth; Pons Varolil. 
| motor to muscles of mastication 
| VI. | Abducent. . Motor to external rectus muscle of Junction of pons and 
| eyeball medulla. 
| VII. | Facial : . Motor to muscles of scalp and face, Posterior border of 
sensory to tongue pons Varolii. 
| VIII. | Auditory . . Hearing and equilibrium . . Posterior border of 
pons Varolii. 
IX. Glosso- Sensory to tongue and pharynx . Medulla oblongata. 
| pharyngeal 
X. Pneumo- _ Sensory to pharynx, cesophagus and Medulla oblongata. 
gastric stomach, and respiratory organs 
