COPULATORY ORGAN'S OF THE AMNIOTA. 
13 
shortened, and, since the penis is not attached directly (except through the ischio- 
cavernosus) to the pelvis, the organ can be protruded. The sphincter cloacae externus 
is strong, and an inner portion of it shows the division into a sphincter aui and a 
sphincter of the opening for the penis. Extraction of the copulatory organ is 
perfected (besides by the relaxation of the M. ischio-cloacalis just described, and by 
contraction of the pelvic portion of the isc-hio-cavernosus) by the assistance of a small 
non-striped muscle, running along the dorsum to the glans penis, and forming the 
continuation of the longitudinal muscular layer of the sinus uro-genitalis. During 
the erection this small muscle acts as M. levator 'penis, with which it is homologous in 
other Mammalia. 
The sphincter ani consequently does not take a share in the muscle-supply of the 
copulatory organ, and thus exhibits a difference from the Birds and Lizards. 
All other Marsupial and Placental Mammalia possess likewise a muscle which arises 
from the pubischium and which grasps the crura penis ; it may, however, arise from 
the ischium or from the pubis only (M. ischio-cavernosus s. erector penis). 
The sphincter ani externus and the M. bulbo-cavernosus are to be looked upon as 
originally one mass, divided, hand-in-hand with the disappearance of the cloaca, into a 
dorsal portion or anal sphincter, and into a ventral portion or M. bulbo-cavernosus, 
5 . accelerator urinse (constrictor vestibuli in the female), with a third portion, the 
M. urethraJis. The M. levator ani, the M. transversus perinei, and the M. ischio- 
cavernosus remind us of the Tiatite arrangement. A M. levator penis is found in 
most Mammals, and is fixed to the dorsum penis. Sometimes^ e.g., in Chiromys and 
in the Marsupials, it arises from the crura penis or from their fascia, and, therefore, 
still retains Monotreme conditions. In most Placentalia it has become attached to 
the skeleton, arising from the symphysis pubis (Bodents, Elephants) or from the 
fascia below it (Monkeys), from the ischial tuberosity (ITedge-hog), from the pubis and 
ischium with a broad origin (Horse), or, lastly, from the first caudal vertebrae, in which 
case it passes to the right and left from the rectum (Carnivora). In Man it is much 
reduced. Its chief action is frequently only the compression of the vense dorsales 
penis. 
Attached to the urethral side of the penis, sometimes extending down to its glans, 
is another, often very powerful, likewise non-striped muscle, the retractor penis, 
fig. 28. It is best developed in those Mammals in which the copulatory organ is 
lodged in a canal formed by the abdominal skin, but it is absent in the Rodents and 
Carnivora, and in those Mammals which have a free or pendent copulatory organ 
(Primates, Chiroptera). 
It arises either from the sacrum or coccyx (Marsupials, Plorse, Boar), and passes in 
this case on each side of the anus, or it comes from the ventral region of the sphincter 
ani (Cetacea, Ruminants); in the Bull it attains a length of about 30 cms., and each 
of its halves is more than 1 cm. in thickness. 
Distinctly copulatory muscles in the Mammalia are consequently derived from 
