Pelvis and Perineum
Pelvis = caudal region of trunk
The pelvis consists of pelvic viscera within pelvic cavity that is bounded on five sides.
Pelvic Viscera:
rectum & anal canal; distal ureters, urinary
bladder & urethra;
male or female genitalia; blood vessels,
lymphatics & nerves
Pelvic Cavity Boundaries:
cranial opening— pelvic inlet, bounded by sacrum, ilium &
pubis (rigid boundary)
cavity walls— osseous pelvis + obturator & gluteal mm. &
sacrotuberous ligament (dog)
caudal boundary — perineum
Perineum = both a region & a wall
As topographic region: from tail to scrotal attachment or entire vulva; between ischiatic tuberosities.
As caudal wall of trunk: muscles, fascia & skin surrounding anal canal and urogenital tract; includes
1. Pelvic diaphragm = levator ani & coccygeus mm. associated
deep fascia
2. Urogenital diaphragm = external urethral sphincter associated
deep fascia 3 External anal sphincter m.
4 Genital striated mm:
male — bulbospongiosus m. & paired ischiocavernosus
mm.
female — constrictor vestibuli, constrictor vulvae &
ischiocavernosus mm.
5. Smooth mm.: rectococcygeus m.& retractor penis m.
Micturition
Anatomy:
Urinary Bladder — apex; body; neck (trigone = region of neck demarcated by ureters & urethra).
Urethra —female: terminates in vestibule;
male: pelvic urethra [preprostatic (cat); prostatic; &
postprostatic regions] & penile urethra
Musculature:
Detrusor m. = smooth muscle coat of apex & body; innervated by pelvic n (S2 +S1 & S3 )
Internal urethral sphincter = smooth m.; innervated hypogastric n. (L2, L3, L4)
female — vesical neck & cranial half of urethra
male — neck (dog); neck & preprostatic urethra (cat)
External urethral sphincter = urethralis m. (striated); innervated by pudendal n. (S2 & S3 + S1 )
female — caudal half of urethra
male — postprostatic urethra
Urine Storage:
Sphincters active via spinal reflexes and detrusor m. inhibited
- internal sphincter exerts tonic activity after bladder is half full
- external sphincter is activated voluntarily or reflexly during phasic pressure increase
Micturition:
Requires prolonged detrusor contraction and sphincter inhibition
- free nerve endings (receptors) —> GVA fibers in pelvic n. —> ascending pathways —> pons —> descending spinal pathways —> activate detrusor & inhibit sphincters
Pain:
GVA pain fibers travel through the hypogastric nerve to reach the spinal cord and brain.
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