618 THE NERVOUS SYSTEM. : 
The dorsal nerve of the penis, the other terminal branch of the pudic nerve, 
accompanies the internal pudic artery beneath the superficial layer of the triangular 
hgament. It passes forward close to the pubic arch, lying beneath the crus and 
erector penis (or clitoridis), and triangular ligament, and upon the compressor 
urethree muscle; piercing the triangular ligament near its apex, at the outer side 
of the dorsal artery of the penis (or clitoris), it passes on to the dorsum of the 
penis or clitoris, to which it is distributed in its distal two-thirds, sending branches 
round the sides of the organ to reach its under surface. In the female the nerve is 
much smaller than in the male. The dorsal nerve of the penis supples one branch, 
the nerve to the corpus cavernosum, as it lies beneath the triangular ligament. 
This is a slender nerve, which, piercing the triangular ligament, supplies the 
erectile tissue of the crus and corpus cavernosum. 
Morphology of the Pudendal Plexus.—The structures occupying the perineum are placed 
in the ventral axis of the body, and comprise from above downw ards the penis and scrotum, or 
mons Veneris and vulva, the central point of the perineum, the anus and ischio-rectal fossa, and 
the coccyx. They are placed on the inner side of the attachment of the lower limbs—the penis 
or mons Veneris in relation to the preaxial border ; the coccyx in relation to the postaxial border 
of the limb. 
The nerves of the perineum, thus reaching the ventral axis of the trunk, are homologous with 
the anterior (ventral) terminations of other nerves. They are separated into two series. Mainly 
supplied through the pudendal plexus by the last four sacral 
and the coceygeal nerves, the perineum is also innervated 
to a minor extent by the first lumbar nerve through the 
ilio-inguinal nerve, which reaches the root of the penis and 
the scrotum. The perineum is thus supplied by two series 
of widely separated nerves, which have their meeting-place 
on the dorsum and side of the penis and scrotum. This 
junction of the ilo-inguinal and pudendal nerves constitutes 
the beginning of the ventral axial area or line, which 
extends peripherally down the inner 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 in- 
Fic. 449.—ScHEME of the innervation nervated by the higher spinal nerves ; the lower parts, by the 
of the hinder portion of the trunk lower nerves. This is best exemplified in the distribution of 
and of the perineum, and the in- the cutaneous nerves. The base of the penis and scrotum 
terruption of the segmental arrange- (or mons Veneris) is supplied by the first lumbar nerve (ilio- 
ment of the nerves associated with jnouinal). The dorsal nerve of the penis (or clitoris), when 
oe onse | hes Ny traced back to the pudendal plexus, is found to come from 
1.10, 11, 12, The areas of distribution the second, and to a less extent from the third sacral nerves ; 
of the lower thoracic nerves ; L.1,2, the scrotal nerves (perineal branches of the pudic and small 
3, The posterior primary divisions scjatic) similarly arise from the third, and to a less extent 
ot the first three lumbar nerves; L.1, from the second sacral nerves; the skin of the ischio-rectal 
The ilio-inguinal nerve; 5.1, 2, 3, fossa and anus is innervated by the inferior hemorrhoidal 
4, 5, 6, The posterior primary divi- ¢ 
aye (6 eee i ee the fourth sacral nerve. The coccygeal plexus, lastly, supplies 
of pudic nerve to penis and scrotum ; the skin round the cocecyx (fourth and fifth sacral and 
S.3, 4, Inferior hemorrhoidal nerve; coccygeal nerves). Judged from its nerve supply the perineum 
4, Perineal branch of the fourth 1s to be regarded as occupying, for the most part, a position 
sacral nerve ; 4,5, 6, Anterior sacro- below or more caudal than that of the lower limb in relation 
coceygeal nerve; D.A.L, Dorsalaxial to the trunk. There is here a remarkable gap in the 
line ; V.A.L, Ventral axial line. 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 middle line of the trunk anteriorly 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 only concerned 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 below the limb. 
DEVELOPMENT OF THE SPINAL NERVES. 
I. Origin of the Spinal Nerve Roots.—The process of development of the spinal 
nerves commences by means of the outgrowth of the dorsal and ventral roots from the 
medullary tube, The two roots take origin in quite different w ays, 
— a _ )~)~— 
(third and fourth sacral nerves), and the perineal branch of 
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