COPULATORY ORGANS OF THE AMNIOTA. 
11 
apparatus. This explains why in the Tortoises defecation and micturition are 
separate acts. How the penis becomes doubled up is not very easily seen ; probably 
through the greater elasticity of its grooved side and through the contraction of the 
walls of the cloacal chamber. 
The vestibulum cloacae is capable of considerable shortening through the longi¬ 
tudinal unstriped muscles of its walls and through the paired M. pubi-caudalis, which 
—arising from the transverse processes of some of the caudal vertebrae near the anal 
opening, and being attached to the splanchnic side of the pubic symphysis—can by 
its contraction bend the tail; at the same time both muscle-bands exert a pressure 
upon the lateri-ventral walls of the outer cloacal chamber. Anal muscles proper, 
like a distinct sphincter, are not developed in the Chelonia. The M. ischiocaudalis 
(flexor caudse ischiadicus of Bojanus) arises from most of the postsacral and caudal 
vertebrae ; its fibres run transversely, and form a broad, although thin, muscular 
layer, which, besides on the tuber ischii, is inserted aponeuroticaliy on the visceral 
side of the ischiadic symphysis. From the caudal corner of this symphysis to the 
anus the muscle-fibres meet with those of the other side, and being partly attached to 
the cutaneous lips of the anus, bridge over the whole cloaca. They act upon the 
latter as a constrictor. Some of the most superficial fibres, between the caudal 
margin of the ischium and the anterior corner of the anus, assume a more longitudinal 
direction and an independent aspect; they are described by Bojanus as dilatator 
cloacae, whilst in Testudo grseca they are hardly developed. 
In the Birds (fig. 15) a typical M. sphincter ani is formed from the same muscular 
layer just described in the Chelonia. This layer is differentiated into two muscles : 
(1) M. transverso-analis; it arises in a variable way from the distal caudal portions of 
the pelvic bones : in rare cases also from the transverse processes of some caudal 
vertebrae ; its fibres form in the medio-ventral line a more or less aponeurotic junction 
with those from the other side, and are also attached to the anterior margin of the anal 
opening, where they are blended with (2) the M. sphincter. The fibres of the latter 
surround the anal opening in the typical way, and have no connexion with skeletal 
parts, except indirectly where there are a few muscular slips attached to the flexor 
muscles of the tail or to the skin in which the tail feathers are lodged. The M. 
sphincter and the M. transverso-analis are, like the Cloaca, supplied by nerves from 
the plexus pudenclus. In Rhea (fig. 15) there is on each side a double M. levator 
ani, which arises from the distal portions of the pelvic bones, and is attached to the 
dorsal corner of the anus. It is probably a differentiation of the M. transverso-analis. 
We can derive the Avian arrangement easily from that of the Chelonia if we 
imagine that in the Birds the M. ischio-caudalis of the Chelonia has lost its origin 
from the tail, and that a separation took place between the anal portion (future M. 
sphincter) and the portion between the anus and the pelvic bones. The rest of the 
reptilian ventri-lateral muscles between tail and pelvis is represented in the Birds by 
the MM. pubi- and ileo-coccygei. 
c 2 
