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THE NERVOUS SYSTEM. 



a special difficulty. In the composition of the brachial plexus the first thoracic nerve is almost 

 wholly engaged in forming the ventral series of nerves. It only gives a minute nerve to join 

 the posterior cord, and this is not always present. In the case of the lumbo-sacral plexus 

 the third sacral nerve does not as a rule divide into ventral and dorsal trunks, but contributes 

 only to the formation of the ventral series of nerves. A solution of this difficulty may be found 

 in the examination of the areas of distribution of the nerves derived from the first thoracic 

 and third sacral nerves respectively. In the case of the brachial plexus (the medial cord of which 

 receives normally the whole contribution of the first thoracic nerve) the medial cutaneous nerve 

 of the arm, the ulnar branch of the medial cutaneous nerve of the forearm, and the dorsal branch 

 of the ulnar nerve supply the dorsal aspect of the limb on its postaxial border. These nerves 

 are in serial homology with the intercosto-brachial and lateral trunks of intercostal nerves. In 

 the case of the lumbo-sacral plexus similarly, in which the third sacral nerve does not divide into 

 ventral and dorsal trunks, the posterior cutaneous nerve of the thigh and tibial nerves containing 

 the contribution from the third sacral nerves innervate, by means of the gluteal and lateral 

 femoral branches of the former and the medial sural nerve (medial cutaneous nerve of the calf) 

 of the latter, the dorsal surface of the limb along the postaxial border, in series with the 

 perforating cutaneous nerve and the perineal branch of the fourth sacral. 



These apparent anomalies appear to indicate that, instead of dividing into its proper dorsal 

 and ventral trunks, the entire contribution of the spinal nerve concerned, is in these instances 

 carried undivided along the postaxial border of the limb in association with the ventral trunks, 

 and that the dorsal subdivisions are thrown off successively as the plexus cords approach the 

 periphery. Indeed, in the case of the posterior cutaneous nerve of the thigh, Eisler has 

 shown that, when the common peroneal and tibial nerves are separated at their origin, its 

 gluteal and lateral femoral branches arise from and are connected with the dorsal trunk, and 

 the perineal and medial femoral branches with the ventral trunk. 



THE DISTRIBUTION OF THE SPINAL NERVES TO THE MUSCLES AND SKIN 



OF THE LIMBS. 



By dissection, experiment, and clinical observation, it is conclusively proved that, 

 as a rule, each nerve of distribution in the limb, whether to muscle or skin, is made up of 

 fibres derived from more than one spinal nerve ; and, further, that in cutaneous distribution 

 a considerable overlapping occurs in the course of the several peripheral nerves. Moreover, 

 the arrangement of the distribution of the nerves to skin and to muscles is not identical. 

 In the case of the skin of the limbs, by the covering of the limb being drawn on to it from 

 adjacent parts in the process of growth, cutaneous nerves are engaged which are derived 

 from sources not represented in the muscular innervation of the limbs. Again, among the 

 muscles, some have undergone fusion, others have become rudimentary, and others again 

 have altered their position in the limb. Bearing these qualifications in mind, it is 

 possible to formulate a definite plan for the innervation of the skin and muscles of the 

 upper and lower limb. The accompanying tables give an analysis of the distribution of 

 the spinal nerves to the skin and muscles of the upper and lower limb respectively : 



I. Upper Limb. A. Cutaneous Nerves. 



1. Dorsal (Posterior) Surface. 



