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Male Reproductive System: External Anatomy

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  • 0:05 Male Reproductive System
  • 1:03 Scrotal Anatomy
  • 1:51 Cremaster Reflex
  • 3:51 External Penile Anatomy
  • 5:43 Internal Penile Anatomy
  • 8:41 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

Learn about the structure and function of the external reproductive structures of the male. This includes study of the scrotal sac, the parts of the penis and the erectile tissues.

Male Reproductive System

Throughout the animal kingdom, all males have one thing in common - they all need a way to spread their DNA, which of course is carried in their sperm. To do this, they have to have specialized external structures, and in most mammals, including humans, these structures include the scrotum and the penis.

Interestingly, while the function across species is the same, the anatomical structure is not always identical. For example: did you know that the male vervet monkey has a blue scrotal sac to aid in the attraction of females, or that the male tomcat has spines on the head of his penis to stimulate egg release in the female? These are just some of the variations seen in male mammals.

Just as the color or structure of each of these has a purpose - the blue in the vervet and the penile spines in the tomcat - the structure of the human male scrotum and penis also has a purpose.

Scrotal Anatomy

We'll start by looking at the scrotum. The purpose of the scrotum, also called the scrotal sac, is to house the testicles. You see, like most mammals, the human male's testicles are located outside of the main abdominal cavity. Why?

Well, the internal body temperature of the average male is higher than the optimum temperature needed for sperm production. So, in order to solve this problem, the testicles relocate to outside the abdominal cavity and into the scrotal sac shortly before birth in a process known as testicular descent.

Each individual testicle is housed in a separate scrotal cavity within the scrotum, separated by a partition. This separation prevents an infection in one testicle from spreading to the other. Now, inside of each cavity we have a serous membrane lining called the tunica vaginalis, which prevents friction between the testes and the scrotum.

Location of the tunica vaginalis
Tunica Vaginalis Diagram

Cremaster Reflex

The scrotum itself is made up of a layer of skin and the underlying layer of smooth muscle known as the dartos muscle and of skeletal muscle known as the cremaster muscle. Remember, smooth muscle is involuntary and skeletal muscle is voluntary. The functions of each of these muscles are to move the testes close to or further away from the body.

Location of the cremaster and dartos muscles in the scrotum
Cremaster Muscle Diagram

Now, thinking back to what we said about the optimum temperature for sperm production, why would we need to move the testes closer to the body? I mean, didn't we say that they're supposed to be away from the body so they can be kept cool? Well, let's think about it this way... how often do you experience changes in temperature? A lot, right? And your body responds to those changes, doesn't it? It responds through either sweating when it's hotter than normal or shivering when you get cold.

Well, the testes need a way of regulating their temperature as well. So, when it gets cold, the muscles in the scrotum contract. This causes the testes to pull in closer to the body in an effort to keep the testicular temperatures from falling too low. This reflex is known as the cremasteric reflex and can be triggered by a number of things. These include sexual arousal, sensory fibers on the upper thigh, or sudden changes in temperature, like jumping into a cold pool.

On the other hand, a rise in air or body temperature can cause these muscles to relax. That allows the testes to move away from the body. The ability of the body to do this is important because it helps to keep testicular temperatures about 2 degrees Celsius lower than your core body temperature, which means that sperm development can proceed normally.

External Penile Anatomy

The second organ we'll discuss in this lesson is the penis. Just as the scrotum has the job of keeping the testicles protected and at their proper temperature, the penis also has a job. The function of the penis is actually two-fold. First, there's the non-reproductive function, and that's the elimination of urine through the urethra.

The second is the reproductive function of sperm deposition into the female reproductive tract. To successfully complete this function, the penis needs to be able to fit into the female reproductive tract. So, the shape or structure of the human penis is made specifically so that it complements the shape of the female.

Think about it this way: If the entrance to the female reproductive tract is a circle and the penis is in the shape of a rectangle or a square, they wouldn't really be very compatible, would they? Like trying to put a square peg in a round hole, it just wouldn't fit right.

With this in mind, we will break down the structure of the penis into its parts. First, you have the root, which attaches the base of the penis to the body wall. Moving up from the root, you have the body, also known as the shaft of the penis. At the top, you have the glans, or the head of the penis, which is expanded and surrounds the urethral opening.

What you don't see on this diagram is a structure that is there at birth but for most boys in Westernized countries is removed in a process called circumcision. Circumcision is the removal of the prepuce (or the foreskin), which is a fold of skin that, at birth, surrounds the glans, or the head, of the penis. Removal of this structure helps in keeping the penis free from bacterial infection as well as from inflammation.

Internal Penile Anatomy

So, now that we've covered the external structure of the penis, what about its internal structure? Specifically, what is it about penile structure that allows it to change from its flaccid (or relaxed) state into an erect state?

The three columns of erectile tissues in the shaft
Erectile Tissue Diagram

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