10 
DR. H. GADOW ON THE CLOACA AND ON THE 
fibres on to the caudal end of the Symphysis ischii and to the neighbouring comer of 
the anal opening. Contraction of this muscle opens the anus. 
The side walls of the anal portion of the cloaca are surrounded by a rather thin 
layer of muscles, which arise in the median line between the right and left halves of 
the M. ischio-caudalis, from some of the first chevron bones of the caudal vertebrae. 
Their fibres are directed more or less transversely towards the margin of the anus, 
where they are attached. The most Superficial portion of this system has, however, a 
more longitudinal direction and is somewhat differentiated off from the rest; its fibres 
meet at the anterior corner of the anus and surround the latter like a sphincter. 
Their contraction closes and elongates the anal slit, whilst through the action of the 
more transverse, deeper mass the anal walls are drawn inwards and are thus likewise 
closed. 
All these muscles, viz., the M. transversus perinei, s. transversus lateralis, s. 
levator, and the M. transversus medianus, s. sphincter, together with the M. rectus 
lateralis, are supplied by a strong branch of the ischiadic plexus. In Alligator 
mississipiensis this nerve-branch comes from the stem a, i.e., the first postsacral spinal 
nerve. It leaves the pelvis laterally from the tendinous insertion of the M. ischio- 
caudalis, between this tendon and the posterior margin of the ischium near the 
symphysis, and then it supplies, situated superficially, the muscles, sending out at the 
same time cutaneous branches. 
Considering the nerve-supply, it seems more reasonable to suppose that the 
sphincter muscles are, at least in the Crocodilia, a differentiation of the post-pelvic 
portion of the system of the M. rectus abdominis, and not of the true caudal muscles. 
In the Chelonia the penis is not attached to the pelvis, nor is it in connexion with 
any anal muscles ; but to its dorsum, not far from the glans, are attached a pair of long 
retractor muscles, which pass laterally from the M. pubi-caudalis (fig. 10) (flexor 
caudse obturatorius, Bojanus) into the pelvis, and arise from one of the most 
anterior lumbar vertebrae. The position and nerve-supply of these peculiar muscles 
show that they are differentiated from the MM. lumbo-caudales (flex, caudae lumbales, 
Bojanus). There is consequently no homology whatever between this Chelonian 
retractor (sometimes called M. ischio-cavernosus) and any of the Avian and Mamma¬ 
lian copulatory muscles. Its function is somewhat complicated. When the penis is 
erected and protruded, the muscle acts as the retractor, but when the organ is in a 
relaxed condition and, when at rest, doubled up and withdrawn into the cloaca, as 
shown in fig. 26, contraction of the muscle, owing to its pulley-like course round the 
M. pubi-caudalis, undoubles it and protrudes it so far that the glans becomes visible 
in the anal opening. This is seen in micturating Tortoises, because the penis hits to 
be straightened in order to withdraw the glans from its position ventrally from the 
fold p, where it blocks the opening of the bladder. During deftecation, on the other 
hand, the doubled halves of the penis are pressed against each other, and, together 
with the fold p, which is likewise pressed down, effectively protect the urino-genital 
