1228 PHYSIOLOGY 



probable that their reaction to this current is to swim * against it (positive 

 rheotaxis), so that they reach the upper part of the Fallopian tubes or the 

 surface of the ovary itself. Fertilisation of the ovum occurs in most cases 

 in the Fallopian tube, and the fertilised ovum is then carried slowly down 

 the tube into the uterus. 



NERVOUS MECHANISM OF IMPREGNATION. Although, in both 

 sexes, coitus is attended by a high degree of psychical excitement, yet it is 

 essentially a spinal reflex, and can be carried out when all impulses from the 

 higher centres are cut off by section of the cord in the dorsal region. The 

 centre presiding over the act is situated in the lumbar spinal cord. The 

 external generative organs, like the bladder, are supplied from two sets of 

 nerve fibres from the lumbar nerves through the sympathetic, and from 

 the sacral nerves. The fibres from the lumbar nerves arise in the cat from 

 the second, third, and fourth, or the third, fourth, and fifth lumbar nerve- 

 roots, and in the dog from the thirteenth thoracic, and the first to the fourth 

 lumbar roots. They run in the white rami communicantes to the sympathetic 

 chain, whence they may take two paths. 



(a) The great majority of the fibres run down the sympathetic chain to 

 the sacral ganglia, whence fibres are given off in the grey rami communicantes 

 to the sacral nerves ; their further course is by the pudic nerves ; none 

 running in the nervi erigentes. 



(6) A few fibres go by the hypogastric nerves to the pelvic plexus. 



Excitation of these fibres causes contraction of the arteries of the penis, 

 nad of the unstriated muscles of the tunica dartos of the scrotum. In animals 

 which possess a retractor penis muscle, excitation of the lumbar nerves 

 causes strong contraction of the muscle. 



The fibres from the sacral nerves can be divided into two classes 

 visceral and somatic. The visceral branches run in the pelvic nerves, or 

 nervi erigentes. Stimulation of these fibres produces active dilatation of the 

 arteries of the penis or vulva, and also inhibition of the unstriated muscle of 

 the penis, the retractor muscle of the penis, when present, and of the vulva 

 muscles. The somatic branches supply motor nerves to the ischio- and 

 bulbo-cavernosi, as well as the constrictor urethra. In the female they 

 supply the analogous muscles, namely, the erector clitoridis (ischio-caver- 

 nosus) and the sphincter vaginae (bulbo-cavernosus). Both these sets of 

 fibres are therefore involved in the erection of the generative organs which 

 accompanies coitus. 



The internal organs, i.e. the uterus and vagina in the female, and vasa 

 deferentia, seminal vesicles, and uterus masculinus in the male, differ from 

 the external organs in receiving no efferent nerve fibres from the sacral nerves, 

 as has been pointed out by Langley and Anderson. They are supplied with 



* Spermatozoa move in a straight line, at the rate of 2-3 mm. per minute. Thus 

 they might traverse the distance of 16-20 cm. between the os uteri and the trumpet- 

 shaped orifice of the Fallopian tubes in three-quarters of an hour. In animals sper- 

 matozoa have been found at the peritoneal end of the Fallopian tubes within an hour 

 or two after coitus. 



