Human/Pig Comparisons

Similarity to human structure

Pigs are mammals. Consequently, all of the major structures found in humans are present in the fetal pig. With proper directions, they can all be readily found, especially with large, full term fetal pig specimens. There are a some differences in structural details, mostly relatively minor in nature. Some examples are given below.

Muscles

In almost every case, fetal pigs have the same muscles as humans, with some small variations in the size and location of some muscles related to the fact that pigs are quadrupedal and humans are bipedal. For example, the major chest and abdominal muscles found in humans are present in the pig. There are some differences in the location of chest muscles that attach to the shoulder girdle. In the hind limb, the pig has the same muscles as humans in the major thigh muscle groups: quadriceps femoris and the hamstrings; see p. 35 of the FPDG (Fetal Pig Dissection Guide). In the hip, however, there are some differences in the gluteal muscles.

Quiz: How are the gluteal muscles of the pig different than the gluteal muscles of the human? See p. 34 of the FPDG.

Internal Organs

Pigs have all of the same thoracic and abdominal organs as humans. There are small differences in a few organs.

Liver – the human liver has four lobes: right, left, caudate and quadrate. The fetal pig liver has five lobes: right lateral, right central, left central, left lateral, and caudate.

Intestines – there is a significant difference in the structure of the fetal pig colon compared to the human colon. The pig colon is spiral. (See p. 45 of the FPDG.)

Adrenal glands – In the fetal pig, the adrenal glands are found near the aorta towards the cephalic end of the kidneys, instead of on top of the kidneys as is the case in humans. (See p. 45 of the FPDG.)

Quiz: Like humans, the adrenal glands of the fetal pig are retroperitoneal. What other abdominal organs are retroperitoneal? (See p. 45 of the FPDG.)

Stomach, spleen, bile duct system, small intestines, kidneys, bladder, etc. – the remainder of the abdominal organs found in the fetal pig are basically the same as found in humans.

Thymus – the thymus is found in the same areas in pigs as in humans. However, it is much larger than most students1 expect. This is not a difference of pigs from other mammals. All mammals have a large (enormous) thymus gland during the fetal stage. It gradually shrinks, relative to the rest of the body, throughout life. (See p. 51 of the FPDG.)

Lungs – Like humans, pigs have multi-lobed lungs.

Quiz: Humans have three lobes in the right lung, two lobes in the left lung. How many lobes are there in the lungs of the fetal pig? (See p. 51 of the FPDG.)

Pericardium, vena cava, esophagus, phrenic nerve, etc. – these other thoracic organs are basically the same in pigs and humans.

Genitourinary structures

Uterus – The fetal pig uterus is of a type called bicornate, compared to the simplex human uterus. This means that the pig uterus has two large horns in addition to the body. These horns are sometimes confused with the much smaller Fallopian tubes. It is the presence of these horns which allows pigs to have a litter of 8 or 10 pigs. (See p. 57 of the FPDG.)

Urogenital Sinus – Pigs have a relatively long urogenital sinus formed by the fusion of the urethra with the vagina. The urogenital sinus then connects to the external genitalia. While humans have a urogenital sinus during embryological development, it is lost except for the vestibule which is considered to be part of the external genitalia. Consequently, in humans the urethra and vagina have separate external openings.

Urethra, ovaries, uterine tubes, labia, mesenteries, testes, epididymis, vas deferens, inguinal canal, prostate gland, etc. – these structures are basically the same in the fetal pig and human.

Blood vessels

Bicarotid trunk – In fetal pigs, the brachiocephalic artery splits into the right subclavian artery and the bicarotid trunk. The bicarotid trunk then splits into the right and left common carotid arteries. Humans do not have a bicarotid trunk; instead, the left common carotid artery branches from directly from the aorta, while only the right common carotid artery originates from the brachiocephalic artery. (See p. 71 of the FPDG.)

Illiac arteries -Humans have a common illiac artery which branches into internal and external illiac arteries. Fetal pigs do not have a common illiac artery. Instead, the internal and external illiac arteries branch directly off of the aorta. (See p. 78 of the FPDG.)

Commonly dissected sheep organs

Sheep heart – The sheep heart is very similar in size and shape to the human heart – slightly larger than the average human heart, but otherwise very similar.

Sheep eye – The sheep eye is identical to the human eye in all major respects.

Sheep brain – The sheep brain has the same basic plan as all mammalian brains, including humans. Compared to a human brain, the sheep brain has a relatively smaller cerebral cortex and consequently a relatively smaller overall size. The other major parts are identical in terms of gross anatomy.

Embryological note

Hemiazygous vein/coronary sinus structure — One of the most notable differences of pigs from humans is in the veins that drain the posterior chest wall. If you look at the back wall of a fetal pig heart, at the location where the coronary sinus is found in a human heart or a sheep heart, you will see a vessel that is enormous compared to the relatively small size of the fetal pig heart. In humans, there is an azygous vein which develops a connection to the heart through the right common cardinal vein (which becomes the superior vena cava in the adult). In humans, the proximal left common cardinal vein becomes very small, draining only the heart wall, as the coronary sinus. The hemiazygous vein develops a connection to the azygous vein through an anastamosis. In pig development, there is no azygous vein, and the hemiazygous vein drains both sides of the posterior chest wall. The hemiazygous vein loses its connection to the right common cardinal vein, and maintains its connection through the left cardinal vein. The left cardinal vein, consequently, drains not only the heart wall, but also the posterior chest wall through the hemiazygous vein. Consequently, the left cardinal vein becomes very large in pigs. In the pig, one could say that the animal has a very large coronary sinus because it drains the hemiazygous vein as well as the chest wall. Alternatively, one could say that in pigs, the proximal hemiazygous vein drains the heart wall.

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