MOEPHOLOGY OF THE LIMB-PLEXUSES. 



741 



the medial side of the lower limb. Apart from this break in their distribution, a definite 



numerical order may be followed in the arrangement of the perineal nerves. The higher parts of 



the perineum are innervated by the higher spinal nerves ; the lower parts, by the lower nerves. 



This is best exemplified in the distribution of the cutaneous nerves. The base of the penis and 



scrotum (or mons Veneris) is supplied by the first lumbar nerve (ilio-inguinal). The dorsal nerve 



of the penis (or clitoris), when 



traced back to the pudendal plexus, 



is found to come from the second, 



and to a less extent from the third 



sacral nerves ; the scrotal nerves 



(perineal branches of the pudendal 



and posterior cutaneous nerve of 



the thigh) similarly arise from the 



third, and to a less extent from 



the second sacral nerves ; the skin 



of the ischio-rectal fossa and anus 



is innervated by the inferior 



hsemorrhoidal (third and fourth 



sacral nerves), and the perineal 



branch of the fourth sacral nerve. 



The ano-coccygeal nerve (coccygeal 



plexus), lastly, supplies the skin 



round the coccyx (fourth and fifth 



sacral and coccygeal nerves). 



Judged from its nerve supply the 



perineum is to be regarded as FIQ. 634. SCHEME of the ianervation of the hinder portion of the 



occupying, for the most part, a trunk and of the perineum, and the interruption of the segmental 



position behind or more caudal arrangement of the nerves associated with the formation of the 



than that of the lower limb in re- limb. 



lation to the trunk. There is here 



a remarkable gap in the numerical sequence of the nerves supplying the ventral axis of the 



body. All the nerves between the first lumbar and the second sacral fail to reach the mid ventral 



line of the trunk and are wholly concerned in the innervation of the lower limb. 



At the preaxial border of the limb (groin) the first lumbar nerve, the highest nerve supplying 

 the perineum, is concerned also in innervating the skin of the limb. At the postaxial border of 

 the limb (fold of the nates and back of the thigh), the nerves which are the highest of those con- 

 stituting the pudendal plexus (the second and third sacral nerves) are also implicated in inner- 

 vating that border of the limb. The fourth sacral nerve is concerned only to a very slight 

 extent in the innervation of the limb by means of the perineal branch, which reaches the 

 beginning of its postaxial border ; the last two spinal nerves are wholly unrepresented in the 

 limb proper and end entirely in the trunk behind the limb. 



The arrangement of the limb nerves is rendered complex and the significance of the plexuses 

 is obscured by the changes through which, coincidently, the nerves, on the one hand, and the 

 parts supplied by them, on the other hand, have passed in the course of development 



Nature of the Limbs. As already described, the mammalian limbs arise as flattened buds 

 from the extremities of the "Wolffian ridge. Each bud possesses a preaxial and a postaxial border, 

 and a dorsal and a ventral surface, continuous with the dorsal and ventral aspects of the trunk 

 and homologous with its lateral and ventral surfaces. Each bud consists at first of a mass of 

 I undifferentiated, unsegmented mesoderm, covered with epithelium. Around the central core of 

 mesoderm which produces the skeletal axis, the vessels and muscles of the limb are formed 

 in situ, the muscles as double dorsal and ventral strata, beneath the corresponding surfaces of 

 the bud. 



Each limb bud is connected to the lateral and ventral aspects of the trunk, and is associated 

 with a number of body segments, varying in the two extremities and in different animals. 

 Although the mesodermal material of which the limb bud is composed exhibits in itself no 

 segmental divisions at any period of its development, a clear indication of the segmental relations 

 of the limbs is obtained from the arrangement of the limb nerves. Taking the nerves which 

 supply the limbs in man as a guide, the segments engaged in the formation of the upper ex- 

 tremity are the last five cervical and first two thoracic. The lower extremity is related by its 

 nerves to all the lumbar and the first three sacral segments. In each limb, the segments at the 

 preaxial and postaxial borders are only partially concerned in limb formation. 



It has been already shown that the somatic branches of the nerves enter the substance of the 



embryonic limb and divide in their course into dorsal and ventral trunks, which supply the 



)rsal and ventral surfaces of the limb bud. The higher nerves supply the preaxial border, the 



lower nerves supply the postaxial border, while the nerves most centrally situated extend furthest 



towards the periphery of the limb. 



In order to understand properly the constitution of the limb-plexuses, it is necessary, further, 

 to make a comparison of the surfaces and borders of the embryonic and adult limbs. 



Upper Limb. (A) Borders. The preaxial border of the upper extremity extends from the 



48 & 



MORPHOLOGY OF THE LIMB-PLEXUSES. 



