382 MORPHOLOGY OF NERVES. 



Nerves of the limbs. As to the morphological nature of the limb-nerves, it is held by 

 Paterson, Eisler, and others that they represent the whole of the anterior primary divisions 

 of segmental nerves, but it seems probable that the view enunciated by Goodsir, namely, that 

 the limb-nerves are greatly developed lateral perforating 1 branches of segmental nerves, is the 

 better founded. At the same time it is to be noticed that the increase of the lateral branch 

 for the supply of the limb is accompanied by a reduction of the other portions of the 

 segmental nerve, which proceeds so far that the anterior and posterior cutaneous offsets, as 

 well as the visceral branch, are all wanting- in the case of the central nerves of the limb- 

 plexuses. 



The transition from the typical segmental nerves to the modified form met with in the 

 limbs takes place gradually, and may be thus traced. The third dorsal nerve has the typical 

 character ; the second differs but little, its posterior cutaneous branch is drawn out over the 

 scapula, and its lateral perforating branch becomes enlarged and extends down the arm. 

 forming a connection with the brachial plexus, while some of the fibres belonging to this 

 division often begin to take a shorter course to the plexus through the communication with 

 the first dorsal nerve (p. 307). In the first dorsal nerve the lateral branch, greatly enlarged, 

 passes directly into the brachial plexus, the anterior and posterior cutaneous offsets are small, 

 and sometimes wanting, and its contribution to the sympathetic is less than that of the 

 second. The eighth, seventh and sixth cervical nerves consist almost wholly of large lateral 

 offsets, the anterior and posterior perforating, and the visceral branches are suppressed, and 

 the supply from the anterior primary division to trunk-muscles is represented only by the 

 twigs to the scaleni and longus colli. With the fifth cervical nerve the posterior cutaneous 

 branch reappears ; and in the fourth, third, and second nerves the lateral offsets are freed from 

 the limb-plexus (except the fourth to a slight extent in some cases), and furnish the super- 

 ficial branches of the cervical plexus, the lower of which are however drawn out over the root 

 of the limb, while anterior perforating branches are wanting. 



At the lower part of the trunk, the twelfth dorsal and first lumbar nerves have the charac- 

 teristic segmental disposition, but their posterior and lateral perforating branches are drawn 

 down over the root of the limb, as is the case also to a slight extent with a part of the anterior 

 perforating branch of the first lumbar (ilio-inguinal nerve), while a small part of the fibres of 

 this nerve often passes into the limb-plexus. The second lumbar nerve is distinctly inter- 

 mediate in character, since it possesses all the branches of a segmental nerve, although its 

 larger part passes into the limb-plexus. Its anterior perforating branch is represented by the 

 genital division of the genito-crural, and its lateral cutaneous offset by the fibres passing to 

 the crural division of the genito-crural and to the external cutaneous nerve. The third 

 lumbar nerve has a posterior perforating offset, but it consists mainly of a large lateral 

 division passing into the plexus ; an anterior perforating branch is wanting, and the contribu- 

 tion to the sympathetic is very small or absent. The fourth lumbar is purely a limb-nerve, 

 the posterior and anterior cutaneous and the visceral offsets being suppressed. The fifth 

 lumbar is like the fourth, except that it may have a small posterior cutaneous offset ; and this 

 branch is regularly present in the succeeding nerves. With the second sacral nerve the con- 

 tribution to the limb-plexus diminishes, the visceral branch reappears, the lateral cutaneous 

 branch begins to be more independent in the small sciatic and perforating cutaneous nerves, 

 and the continuation of the trunk to form an anterior perforating offset is to be recognized in 

 the pudic nerve. The third sacral furnishes only a small contribution to the limb-plexus, and 

 both it and the fourth sacral have the usual constitution of segmental nerves, but their 

 anterior primary divisions are more or less closely united in the pudic nerve. Of the branches 

 of the latter, the inferior hasmorrhoidal probably corresponds to a lateral perforating offset, 

 while the perineal division and the dorsal nerve of the penis represent anterior perforating 

 offsets. 



In the foregoing sketch it has been shown that the lateral divisions of the nerves are 

 enlarged and carried outwards to supply the integument of the limbs. The cutaneous offsets 

 of some of the posterior primary divisions are also drawn out to a slight extent over the roots 

 of the limbs (especially the lower), but they do not enter into the plexuses. The anterior 

 perforating branches however are not extended to the limbs, except the first lumbar to a slight 

 degree where the lower limb impinges upon the area of these nerves. The supply of the 

 muscles of the limbs by lateral divisions of spinal nerves offers no difficulty, since these 

 muscles are derived from the superficial layer of the great ventro-lateral muscle (see Vol. II, 

 p. 274), and the obliquus externus, the sole trunk-muscle of this layer (the pectorales, serratus 

 magnus, latissimus dorsi, <fcc., being limb-muscles, and deriving their nerves from the limb- 

 plexus), is supplied by the lateral cutaneous branches of intercostal nerves. 



The number of nerves entering the limb-plexus is subject to some variation, not only in 

 different animals, but also in different individuals of the same species. In man, the 'brachial 

 plexus is formed mainly by the lower four cervical and first dorsal nerves, but contributions 

 are often furnished by either or both of the fourth cervical and second dorsal : the number 

 varies therefore from five to seven. The lower limb-plexus comprises the greater part, but 



