Besides the sphincter pupillae, the fibres of the ciliary muscle are stimulated.
Both first and second compartments are remarkable for the presence of a number of pouches or cells in their walls, with muscular partitions, and a sphincter-like arrangement of their orifices, by which they can be shut off from the rest of the cavity, and into which the fluid portion only of the contents of the stomach is allowed to enter.
The mouth consists of two portions, an outer vestibule and an inner apertura oris; the latter is surrounded by a sphincter muscle, which forms the so-called velum.
The hind-gut is also provided with sphincter and dilator muscles, and these may produce rhythmic expansion and contraction, causing an inflow and outflow of water through the anus, which has been supposed to aid in respiration.
The iris contains a sphincter and a dilator muscle; the former, supplied by branches from the oculomotorius nerve, is under control of the will, whilst the dilator fibres belong to the sympathetic system.
In the Entoprocta the tentacles are withdrawn by being infolded into the "vestibule," a depression of the oral surface which can be closed by a sphincter muscle.
This can be experimentally shown - by the method of exclusion - to be caused by a paralysis of the terminals of the third cranial nerve in the sphincter pupillae of the iris.
The third or outermost chamber, the proctodaeum, is closed externally by the sphincter ani; the orifice is quite circular.
Posterior superior iliac spine Cut end of rectum Apex of sacrum Great sciatic notch Ureter Peritoneum Spine of ischium Bladder wall Seminal vesicle Tuberosity of ischium Ischio-rectal fossa Cut end of rectum External sphincter ani Gluteus maximus better seen in young the prostate the urethra runs more forward for about threequarters of an inch, lying between the two layers of the triangular From C. S.