9.27.96

refs:
dyce p 186-197
grandage, the erect dog penis: a paradox of flexible rigidity; vet rc 
8/8/95 pp 141-147


"a topic of intrinsic interest to all of us" - dr. morrison.

handout: male urogenital system, copie of above referenced article, and 
fig 9-35, from "viscera of domestic animals" on reserve in library.

be aware that in prenatal development the testis starts in the cranial 
sublumbar region, before migrating through the inguinal canal into the 
scrotum. so the vessels and nerves of the testis come from a different 
source than those of the penis and the prepuce and what have you.

in the cat, the testis is very high up under the anus and the penis is 
projecting caudally. how do cats copulate w/the penis pointing caudally? 
Well, there is an inelastic ligament along what become the dorsal surface 
of the penis as it becomes erect, so the penis curves around 
ventro-cranially as it expands and ends up facing forwards.

in the dog the testes are more forward. boar somewhere in between. in the 
bull they are really under the body, more, and the head of the epidydimis 
is oriented differently than in the dog. see handout.

scrotum innervated by genital femoral nerve, testes by testicular nerve.

in the case of the accessory sex gland, where the dog only has a prostate 
gland (important for pH, antioxidant activity, nutrients for sperm), the 
cat also has a small bulbourethral gland, and the bull also has seminal 
vesicles that dogs don't have, and a bulbourethral gland, and the 
ampullary gland (stallion has ampullary gland too.) neutered animals have 
these glands atrophy. the accessory sex glands are responsible for the 
bulk of the semen. 

bull: 5cc
stallion: 100cc
dog: 30cc
boar: 200-250cc -->can take 20 min or so to finish ejaculating. a very 
relaxed sort of event called "soaking" where the bull just lies on top of 
the sow resting there and contracting pelvic muscles gently.

on to the prepuce. we've seen this in the dog and we know that it's a 
sheath encasing the penis and is attached to the ventral body wall. all 
of the animals commonly handled have a similar arrangement, except in 
primates, where it isn't attached to the body wall, so the penis hangs 
down. it's a simple sheath in most animals, although in the stallion 
there is some specialization. [slide: saggital section of horse penis]. 
there are two layers to the prepuce of the stallion, an external and 
internal layer. that gives a lot of slack for the penis when it becomes 
erect and quite long, so everything can stretch out. it's sort of like a 
sharpei, there's quite a bit of extra skin. 

in the dog, the opening of the urethra at the end of the penis is just a 
slit, but some animals have specialized openings. in the horse there is a 
urethral process which opens into a fossa with an associated diverticulum 
which can collect various secretions and desquamated cells (smegma). This 
is called the "bean" by "horse people" according to dr.morrison...

[slide: equine penis] dr morrison mentioned again that the equine penis 
is quite larg.

note ring of smooth tissue on flaccid penis is the inner preputial ring.

innervation of portion of prepuce lying on shaft of penis is innervated 
by pudendal nerve/dorsal nerve of penis (as is the penis itself). the 
part of the prepuce that is attached to the body wall and isn't attached 
to the penis directly is innervated by caudal lumbar nerves. there are 
preputial muscles, very well developed, that pull the prepuce and keep it 
covering the end of the penis, and are innervated by the brachial 
plexus/lateral thoracic nerve. preputial muscle plays an important role 
in erection in the dog.

now we'll move on to the penis (he said it, not me) and mechanism of 
erection:

remember that the urethra is running over the brim of the pelvis and has 
to make the bend and as you know the extra pelvic part of the urethra is 
supported by the two cavernous bodies, the corpus cavernosum and corpus 
spongiosum. the penis is attached to the ischium by two crura, which 
attach to the tuber ischiae and contain the spongy tissue which contains 
the blood when the penis becomes erect. and then there is in the pelvic 
urethra a little bit of spongy tissue around there that helps to keep it 
patent by providing sort of something holding it but when it comes around 
the brim of the pelvis it is initially bilobed and that is called the 
bulb of the penis, the urethral bulb, and all of the crura and the bulb 
combined are called the 'root' of the penis.
in the case of larger animals there is a suspensory ligament from the 
ventral surface of the pelvis to hold the penis in its horizontal 
position, but dogs don't have this ligament. so, the two crura which 
you've already seen, extend and the corpus spongiosum surrounds the 
urethra and you have the corpa cavernosa on either side supporting the 
urethra which lies ventrally but the bulb and the corpus spongiosum are 
of course confluent and it is these structures which have to fill with 
blood. there are variations in the blood supply which are apparent in the 
stallion.

in the bull you have a simple arrangement of three arteries supplying the 
penis branching off the artery of the penis. the artery of the bulb goes 
into the bulb and supplies the corpus spongiosum which is confluent with 
the bulb, and there is a branch called the deep artery of the penis, and 
there is the dorsal artery of the penis which supplies the cavernosum and 
continues out to supply surface structures and the more distal part of 
the penis and communicates with the other arteries as well.

in the dog it is mostly the same...artery of bulb, deep artery, and 
dorsal artery, but also some communication from the external pudendal 
which anastomoses with the dorsal artery distally to some extent. mostly 
like bull though.

stallion, you've got an awful lot of penis to supply blood to so the 
internal pudendal supplies the artery of the bulb of the penis, and a 
little bit of deep artery comes off it, and then the obturator artery 
comes out the obturator foramen and provides a middle artery of the 
penis, which is only found in the stallion, and it has caudal and cranial 
branches that in effect act like the dorsal artery of the penis. there is 
an anastomosis off which a branch goes to become the deep artery. (he's 
losing us here). again, the middle artery anastomoses with the external 
pudendal, not directly - but they communicate. so the horse has an 
additional supply of blood and this is important to know if you need to 
amputate a horse penis due to cancer or something. but it's simpler in 
the dog, with the supply mainly from the internal pudendal with a small 
contribution from the external pudendal.

nerve supply - sensory nerves to the penis = dorsal nerve of the penis. 
comes from the pelvic plexus . both sympathetic and parasympathetic 
nerves supply the smooth muscle of the penis. sympathetic outflow has to 
be reduced to allow erection so that the small arteries will relax.

like many things in veterinary anatomy, there are benefits and 
nonbenefits to having to learn different animals but you find some 
interesting features when you do this. in the boar, you see the penis has 
a spiral. and the preputial cavity has a BIG diverticulum that fills up 
with lots of smelly gunk and can be surgically removed in stud boars to 
help people deal with them.

covering the crura of the penis is the ischiocavernosus muscle, inserts 
on the corpus cavernosa. surrounding the c.spongiosum is the 
bulbospongiosus muscle, which you've also seen (he's making a lot of 
assumptions here.)
the ischiocavernosus muscle contracts rhythmically and helps move semen 
along to the distal end of the penis. the bulbospongiousus muscle also 
helps semen along its way by contracting during ejaculation.

[slide-dog butt] coming from tuber ischiae is a small muscle called 
ischiourethralis, which inserts on a fibrous ring surrounding the dorsal 
vein of the penis. when the animal is copulating/erect, the muscle 
contracts tonicly, squeezing down on the dorsal vein of the penis. this 
tiny muscle is important because the tie won't occur if this muscle isn't 
working because the penis won't become fully erect.

in the case what we have actually are three different kinds of penes 
(penes?). we've already seen the kind of penis which includes an os 
penis, which is really a distal continuation of the corpi cavernosa. 
(does he mean proximal?). then in the boar and the bull you see the 
fibroelastic kind of penis, which has a lot of fibrous tissue, doesn't 
increase in diameter when erect, just gets very hard; and in the horse 
there is the musculocavernous kind of penis which needs a LOT of blood to 
fill it up to become hard, and has a tunica alberginia (SP??) forming a 
vascular splint kind of thing for the penis. the ram or the buck has a 
long thin urethral process that sometimes fills up with calculi in a 
pathology called "pizzlerod" (SP?).

in the horse, the spiral helicine arteries, with relaxation of smooth 
muscles, relax and fill up and allow erection. the musculocavernosus 
penis also has at the end a bulbospongiosum which enlarges to form the 
glans, which is big and shaped differently than in the dog. when the 
animal dismounts the corona of the glans flares out. in the case of the 
bull, think of a garden hose, flexible, and what happens when you turn on 
the water? so, blood rushes in there, measured up to 14,000 mm/Hg!! 
sometimes, there are blowouts, and you can see hematoma of the penis in 
the bull. 

retractor penis muscle, smooth muscle, comes from caudal vertebrae, 
swings around anus and lies on caudal surface of penis. well developed in 
bull and boar although present in all animals. 

in the case of the dog you have of course the os penis, the c.cavernosum, 
and the glans is divided into the bulbus and the pars longus. in the 
erect dog penis the  ischiourethralis clamps down, keeping blood in.

look at dyce fig. 9-35 showing flaccid, engorged and intromitted penis.