Most of the clitoris is hidden from view inside the body. All we can see and touch are the glans, and to a varying degree, the projecting segment of the body, under their concealing layer of soft tissue. The following images and video provide us with a view of what is hidden inside.
All the internal clitoral structures, body, crura, and bulbs, are erectile in nature . The crura extend into and become conjoined within the body, where they remain separated by a layer of fibrous tissue, a septum. Erectile tissue contains hollow spaces that fill with blood during sexual arousal. If erectile tissue is contained within a tissue layer that doesn't stretch, it becomes firm to the touch during sexual arousal. Envision multiple empty balloons inside an otherwise empty cylinder shaped plastic bag. Now envision the balloons filling with water, causing the plastic bag to expand and become rigid.
The body and crura contain erectile tissue that is surrounded by a layer of constrictive tissue that allows them to become firm, 'erect', during sexual arousal . The bulbs contain erectile tissue but a less constrictive outer tissue layer, which means they can become 'engorged', but not erect . The erectile structures of the penile shaft are contained within two layers of constrictive tissue, which allows it to become truly erect, whereas the clitoris is believed to only become firmer . The fully 'erect clitoris' probably is not as firm as a fully 'erect penis'. It is unknown if this is equally true of clitorises that are highly developed***.
The clitoris is prevented from pointing upwards during sexual arousal by its suspensory ligament . Based on some anatomy illustrations, the suspensory ligament and tendons of the bulbospongiosus and ischiocavernosus muscles may limit or prevent access to the clitoral body during sexual stimulation. Those structures are not shown here, to allow clear representation of the internal clitoris. Each woman needs to determine for herself how accessible her clitoris is, and demonstrate this to her sexual partner.
The clitoral glans contains 'minimal spongy tissue' that is not erectile in nature , it corresponds to the corpora spongiosum of the penile glans. The amount of spongy tissue is dependent on the level of clitoral development. The glans is rich in blood vessels, but has no cavernous areas. In all clitorises, the erectile structures of the body, the paired corpora, project slightly into the glans , and may result in the glans feeling firm to the touch during sexual arousal. The inner labia are made up of the same tissue as the glans, and both fill, become engorged, with blood during sexual arousal.
The 3D illustrations presented here are based on MRI images of a single clitoris. The appearance of the clitoris, and its size and structure, vary greatly between individuals, depending on factors like genetics, age, presence of chronic disease (diabetes and cardiovascular), and hormone levels . 'The erectile tissue structures [are] more bulky and more easily defined in...younger' women . A woman, or her partner, should not expect her anatomy to be exactly as shown.
The size of the internal clitoral structures vary significantly between women, with the largest being more than twice that of the smallest. The body's width varies from 0.4 to 0.8 inches (1 to 2 cm.) and the length from 0.8 to 1.6 inches (2 to 4 cm.) . The crura are 2.0 to 3.5 inches (5 to 9 cm.) long . The bulbs are 1.2 to 2.8 inches (3 to 7 cm.) in length, and vary in shape from 'crescentic' to 'triangular' . For some women, the bulbs are more closely related to the clitoris and urethra than the vagina . The varying size and shape of these structures is demonstrated in two photographs shown below. Given the amount of anatomical diversity, should we expect all women to have identical sexual responses and experiences?
The clitoral glans and hood, and the soft tissue covering the body (highlighted in red in the following photo), are part of the somatic nervous system and sensitive to the full range of stimulation. The projecting portion of the clitoral body has somatic sensory nerve endings located on the surface of the erectile structures that are sensitive to pressure and vibration, and a woman may feel her clitoral body become engorged with blood , and perhaps pulse and throb. The nerve endings are located along the top two-thirds of the clitoral body, adjacent to the path of the dorsal nerve and its lateral offshoots .
The nerves internal to the erectile structures, body, crura, and bulbs, are part of the autonomic nervous system and described as 'microscopic and difficult to define consistently' , and 'few and thin' . These nerves are associated with the smooth muscle fibers and blood vessels responsible for erection . This likely means the erectile structures are, at most, sensitive to pressure and pain. While not shown here, the crura and bulbs are covered by a thin layer of muscle****, and other tissues , which prevents them from being directly stimulated. In the photograph shown below on the left, observe how the bulb on the right (under the word "urethra") remains concealed by tissue, as would be the case in a living woman. The internal erectile structures don't have nearly as many nerve fibers as the glans . The bulbs have even fewer nerve ending than the body and crura .
"[T]he female CN [cavernous nerve] [to the crura] contains no sensory fibers." 
"The sensory function of the clitoris may be exclusively supported by the…somatic nervous system, through PN [pudendal nerve] branches, including the DCN [dorsal clitoris nerve] [to the clitoral glans and body]." 
"Increased density of small nerves in the glans suggests this is the location of heightened sensation. Decreasing quantity of nerves in segments closer to the urethra may indicate these zones are less important for sexual sensation."
It seems unlikely, from an anatomical perspective, for the internal erectile structures of the clitoris to be major erogenous zones. The clitoris has thousands of nerve endings*, but they are located within the glans, not the internal structures. The somatic nerves of the clitoral body are located along the side opposite the vagina. There is little or no reason to believe stimulation of these internal clitoral structures during vaginal stimulation should result in a "clitoral orgasm." The sensitivity of the front wall of the vagina, the location of the G-Spot, is unrelated to these deeper clitoral structures, as the bulbs are located above the urethra, not within the vaginal wall.survey addressing vaginal sensitivity doesn't allow women to indicate if this specific situation is true for them, only allowing them to indicate whether the front OR sides are sensitive. But, I don't recall anyone stating this was true, and generally the media is quick to point out vaginal erogenous zones. Since the majority of women indicate their vagina is less sensitive to stimulation than the external clitoris, this would appear to indicate the internal clitoris isn't a common and/or major erogenous zone. While unlikely, it is still a possibility, as some women indicate their cervix is an erogenous zone, despite being an 'internal' reproductive organ.
The images shown above provide an indication of why vaginal intercourse often times doesn't result in orgasm for women. Keep in mind, the only portion of the clitoris sensitive to soft touch and frictional stimulation is the relatively tiny glans, which is located up and away from the penis during intercourse, not to mention covered by a protective hood. The penis may apply pressure to the erectile structures of the clitoris, but from an anatomical perspective, there is little to no reason to believe this should result in vaginal orgasm.
For men, and women with access to a penis, studying the much larger and prominent penis will provide some indication of the sensitivity of the various portions of the clitoris, as '[c]litoral and penile histology [microscopic structure] appears very similar' . An uncircumcised penis more closely resembles a clitoris than a circumcised one. Consideration of how men masturbate, and enjoy having their penis manually and orally stimulated, will provide some indication of how to stimulate the clitoris. In general, men find the glans, and area surrounding its junction with the shaft, the most sensitive, and this appears to be equally true of women.
The major constraints with this advice are, many if not most women do not have an accessible shaft to their clitoris, and their clitoral glans tends to be even more sensitive to stimulate than the penile glans, as a result of the same, if not greater**, number of nerve endings being contained in a much smaller area. In addition, while the penis can be grasped in the hand, the clitoris can only be caressed with the fingers. The best way to learn how a woman likes to stimulate her clitoris is to watch her masturbate to orgasm, and then have her guide your hand with hers, as she teaches you what works best for her. A woman's masturbation technique(s) provide a good indication of which areas of her body are sensitive to sexual stimulation, and as a result are erogenous zones.
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** While popular texts and websites frequently state the clitoris has more nerve endings than the penis, I have not found a medical reference that substantiates their statements. A medical article titled Cutaneous Corpuscular Receptors of the Human Glans Clitoris: Descriptive Characteristics and Comparison with the Glans Penis published online in May 2013 states "Some authors have claimed that the clitoris is the most densely innervated part of the human body, but this claim is difficult to confirm, as there is no universally accepted method of quantifying the density nerve fibers and corpuscles in the human skin."
*** The largest 'clitorises' appear to occur concurrently with Partial Androgen Insensitivity Syndrome (PAIS), meaning the person is intersexed, and they may see themselves as male, female, both, or neither, and having a penis or clitoris.
**** The muscles covering the clitoral bulbs and crura contract automatically as a result of stimulation of the area between the anus and urethral opening . This may apply pressure to these structures, which a woman may or may not be aware of. It seems possible for women to perceive this as pressure or swelling in the area of the vulva.
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