Not all humans have the same muscles.  Some people lack individual muscles which most humans percent.  The muscles which may be absent includes the risorious, platysma, stylohyoid, sternohyoid, omohyoid, splenius, spinalis cervicis, intercostals intimi (in the upper portions of the thorax), serratus posterior superior, serratus posterior inferior, xiphoid slip of the diaphragm, pyramidalis, coccygeus, occipital attachment of trapezius,  coronoid attachment of pronator teres, flexor carpi radialis, palmaris longus, flexor pollicis longus, brachioradialis, extensor digitorum minimi, extensor pollicis brevis,  psoas minor (in 40%), sartorius, gemellus superior, quadrator femoris, semimembranosus, peroneus tertius, gastrocnemius (the entire muscle or just the lateral head), plantaris, and the flexor digitorum accessorius (Gray, ).

     Some people possess a pair of muscles where most humans possess only one.  The individual muscles which may be duplicated includes the risorious, stylohyoid, sternohyoid, omohyoid, rectus capitis major, rectus capitis minor, pyramidalis, palmaris longus, brachioradialis, extensor digitorum, pectineus (each of its two layers may have a different nerve supply), adductor longus, semimembranosus, and the extensor digitorum longus (Gray, ).  Humans sometimes have a fourth scalene muscle (Hartman, 1933).

     Even though two humans may possess the same muscles, there can be variations in the precise sites of the muscle attachment.  The list of muscles whose sites of attachment is known to vary in humans includes the risorious, stylohyoid, scalenes, longus colli, splenius, longissimus, spinalis, rotators thoracis, rotators cervicis, interspinales, transverses thoracis, trapezius, latissimus dorsi, levator scapulae, pectoralis major, pectoralis minor, subclavius, serratus anterior, deltoid (the tendon may actually reach the forearm), biceps brachii (it may possess a third head exists in 10% people), flexor carpi radialis, palmaris longus, flexor digitorum superficialis, brachioradialis, extensor carpi radialis brevis, psoas major, sartorius (it may have 2 heads), rectus femoris, biceps femoris (it may have only one head), tibialis anterior, gastrocnemius (may have a third head), politeus (may have an additional head), and tibialis posterior (Gray, ).  One anomaly of the soleus in humans is an enlarged fibular head which reaches the head and neck of the fibula, a condition known in other primates.  Some humans lack a soleus or possess a soleus which is fused to the gastrocnemius or completely separate with a separate insertion on the calcaneus (Barberini, 2003).

      Some people have a single muscle which has resulted from the joining of muscles which exist separately in most people.  The number of muscles which may fuse with other muscles includes the mylohoid, geniohyoid, omohyoid, pectoralis major (to the deltoid and across the sternum to each other), serratus anterior, deltoid, infraspinatus, teres minor, brachialis, extensor digitorum minimi, anconeus, extensor pollicis brevis, tensor fascia latae (fibers may reach femoral condyle), articularis genus, adductor brevis, adductor magnus, extensor hallucis longus, and peroneus brevis.   Some human variations include the opposite condition in which a muscle which is a single structure in most people has divided into separate muscle.  The number of muscles which may split into separate parts includes the trapezius, pectoralis major, pectoralis minor, deltoid,  brachialis, and adductor brevis (Gray, ).

      Some people possess a muscle which includes a supplementary portion not found in most people (referred to as an additional muscular slip).  The list of human muscles which may occur with additional slips include the latissimus dorsi (this slip may be double), deltoid, subscapularis, teres minor,  supraspinatus, brachialis,  flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, extensor digitorum, extensor indicis, gluteus maximus, gluteus minimis, biceps femoris, extensor hallucis longus, extensor digitorum longus, flexor hallucis longus, extensor digitorum brevis, and the flexor digitorum accessorius (Gray, ).  

     Some people possess a muscle which most humans lack (although very often these muscles are known in other primates).  Some muscle which appear only as occasional variants include the cleidohyoid, sternalis, extensor indicis brevis manus, and abductor ossis metatarsi digiti (Gray, ).  In some, the plantaris is anamolous and forms an accessory soleus (Barberini, 2003). A number of anomalous hand extensor muscles are known such as the anomalous extensor indicis proprius, extensor digitorum brevis manus, extensor medii proprius, and extensor indicis et medii communis.  While many are asymptomatic, some create problems such as joint pain (Tan, 1999).   The pyramidalis may be absent in humans, apes, and some monkeys.  Most humans and apes lack sternocostal muscles (Hartman, 1933).  The omohyoid has 3 bellies and abductor pollicis brevis forms separate slips  in some gorillas and chimps. (Gibbs, 2002).   

     The pectoralis abdominis and panniculus carnosus which are present in Old World monkeys are absent in humans.  The panniculus gradually decreased in higher primates.   In humans a few axillary slips and an occasional sternalis muscle over the pectoralis are remnants of this ancestral muscle. (Weichert, 1970, p. 528; Hartman, 1933).  The levator claviculae, which exists in many monkeys and apes, is occasionally found in humans (Hartman, 1933).  The biceps brachii may have accessory heads.  In apes and humans, only the superficial part of the coracobrachialis is usually present. (Hartman, 1933).  The brachialis originally possessed two heads and some vestigial remnants of this pattern can be seen in most primates. (Hartman, 1933,p. 133).  The dorso-epitrochlearis is present in lower primates, is occasionally absent in apes, and is usually absent in humans (Hartman, 1933).

In apes and occasionally in humans, the flexor carpi radialis has an origin on the radius.  Apes and humans usually lack the epitrochleo-anconeus of lower primates, which was originally derived from the flexor carpi ulnaris (Hartman, 1933, p. 137).  A deep head of the pronator teres occurs in apes and humans but not lower primates (Hartman, 1933, p. 138).  Chimpanzees and humans frequently possess a coronoid origin of the flexor digitorum sublimis; apes are the only primates with a radial origin of this muscle (Hartman, 1933, p. 138).  The flexor digitorum profundus may have an origin from the sublimis in lower primates and occasionally in humans (Hartman, 1933, p. 138).  The ancestral radial extensor is divided into brachioradialis, extensor carpi longus, extensor carpi brevis, abductor pollicis longus).  Only higher apes possess an extensor pollicis brevis (Hartman, 1933, p. 141).  In African apes, the deep extensor layer is reduced (Hartman, 1933).   Apes may lack palmaris brevis (Hartman, 1933, p. 148).

Although the peroneus tertius has frequently been described as a muscle which only exists in humans as an adaptation to bipedal locomotion, it occasionally occurs in gorillas and rarely in chimps (Hartman, 1933, p. 166).



     Not all humans have the same cardiovascular system.  There are a number of variations which can occur.  Heart abnormalities known in humans include the absence of a heart, the heart angled on the right half of the body, a split atrial septum, persistence of a truncus arteriousus, transposition of the great arteries, persistence of the left superior vena cava, duplication of the vena

more pulomonary veins emptying into a systemic vein, and the presence of an obturator artery.

     Congenital cardiac defects occur in 1% of live births and an estimated 10% of stillbirths (Schneider, 1999).  There are a number of problems which can result from the unequal division of the ventricles (interventricular septum defects) or of the truncus arteriosus.  The division of the truncus can be unequal in the tetralogy of Fallot.  Eight in 100,000 births possess a persistent truncus arteriosus (Sadler, p. 200-4).


A patent ductus arteriosus is one of the most common defects of the great vessels of the heart (Sadler, p. 212).

     Not all humans possess the same blood vessels.  Some blood vessels may be absent including the brachiocephalic trunk (the right common carotid and subclavian remain separate), common carotid artery (internal and external carotids remain separate), external carotid artery, internal carotid artery, mastoid branch of occipital artery, stylomastoid branch of occipital artery, frontal and parietal branches of the superficial temporal artery, connecting arteries of the cerebral arterial circle, posterior inferior cerebellar artery, second posterior intercostals artery absent, brachial artery (the axillary divides into the radial and ulnar arteries), celiac artery,  supraduodenal artery,  posterior gastric artery, femoral artery (replaced by inferior gluteal artery), and the anterior femoral cutaneous vein (Gray, ).   The azygos vein is variable in its tributaries and can sometimes be absent (Arslan, 2000).

     Some people possess a duplication of a blood vessel which is single in most humans.  The blood vessels which are known be double in some people include the left posterior interventricular artery (or triple), connecting arteries of the cerebral arterial circle (or triple), supraduodenal artery, external jugular vein, great saphenous vein, inferior vena cava, renal vein, and accessory hepatic veins.  Some blood vessels may form additional branches in some people such as the right coronary artery (Gray, ).

     Although two people may have the same arteries and veins, they may not always service the same anatomical regions.  Some blood vessels may vary with respect to their origin (arteries) or their destination (veins) including the conus artery, artery of the SA node, coronary arteries, right common carotid artery (which may arise directly from arch of the aorta), right sublavian artery (which may arise directly from the arch of the aorta), left common carotid artery (which may arise from brachiocephalic trunk), vertebral arteries (which may arise from arch of the aorta), internal carotid arteries (which may arise from arch of the aorta), bronchial, thyroidea ima, superior thyroid arteries, a branch of the sternocleidomastoid artery, lingual artery, pterygoid branches of the maxillary artery, labyrinthine artery, radial artery, phrenic arteries,  accessory left gastric artery, hepatic artery, gastroduodenal artery,  cystic artery, superior gluteal artery, inferior epigastric artery, arteria profunda femoris, peroneal artery, and the left common iliac vein (which may join vena cava as high as the kidney).  Some veins vary in their connections such as the deep temporal veins, retromandibular vein, occipital vein, pharyngeal vein,  cranial dural venous sinuses, right posterior intercostals vein, azygos vein, hemiazygos vein, ascending lumbar vein, brachiocephalic vein (which can empty into right atrium), and cystic veins (Gray, ).  Only 65% people have normal branching from the arch of the aorta.  Sixty percent have ‘anomalous’ Circle of Willis and in 10% not complete of people the circle is not complete (Gray, ). 

     Some blood vessels vary in number in humans including the right posterior ventricular rami, interventricular rami, posterior interventricular arteries, right and left ventricular rami, left diagonal artery (while most people have none, some have one or two),  coronary arteries, glandular branches of the cervical branch of the facial artery, dorsal lingual branches of the lingual artery, pharyngeal branches of the ascending pharyngeal artery, pterygoid branches of the maxillary artery, anterior cerebral arteries, orbital arteries, temporal arteries, anterior communicating arteries, branches of the axillary artery, muscular branches of the brachial,  esophageal arteries,  super pancreaticoduodenal arteries, segmental branches of splenic artery, pancreatic branches of splenic artery, short branch of splenic artery, branches of the renal artery,  branches of the tarsal arteries, branches of the lateral plantar artery, muscular branches of the politeal artery, pulmonary veins, accessory lobar veins, anterior cardinal veins, vena cordis minimae, cranial dural venous sinuses, emissary veins, and the short gastric veins (Gray, ).  Approximately half of human embryos possess an anastomosis between the inferior epigastric artery and the obturator arteries (Funke, 1998).

     Some new variant blood vessels may form in some people which are not typical in humans such as an ascending aorta, a pair of descending aortas which later fuse (as in reptiles), a left brachiocephalic trunk, linguo-facial trunk, transverse cervical artery, 2 branches of radial artery, vasa aberrantia between brachial and axillary arteries, accessory hepatic arteries, accessory renal arteries, accessory pudendal arteries, 2 branches of the femoral artery, accessory vertebral vein, accessory hepatic veins, accessory saphenous vein, and a common trunk for the superior gluteal veins (Gray, ).  Tree shrews and primates lack an artery poplitea profunda although the embryos of both tree shrews and humans possess this artery (Funke, 1998).  The artery ischiada supplies the leg in all vertebrate embryos but is usually absent in adult amniotes.  There are rare cases of its presence in humans (Funke, 1998).

     Veins can possess valves.  The number of valves may vary, such as in the popliteal vein, and be present in veins which lack valves in most humans, such as the ovarian veins.  The hepatic portal vein and its tributaries lack valves although these valves are present in human fetuses.  Some adults retain vestigial veins.

A large number of additional abnormalities are known in humans, including a double aortic arch. 

In some people, there is a double inferior vena cava starting in the lumbar region because the left sacrocardinal vein is still connected to the left subcardinal vein.  Some people lack an inferior vena cava because the right subcardinal vein (what should be the renal segment of the inferior vena cava) does not fuse to the hepatic segment but instead enters the superior vena cava by way of the supracardinal azygos) vein.  Other abnormalities include a double superior vena cava and a vena cava which is derived from the left cardinal system rather than the right (Sadler, p. 219-220).


The following images are of abnormal blood vessels in cats.

cat vessels



     Unfortunately, the reorganization of the early pharyngeal arches does not always occur properly.  Some individuals have cysts which develop at the sites of the first pharyngeal grooves.  These may form branchial sinuses which open to the outside of the throat or rarely, they may open into the pharynx.  (Moore, p. 227).  Abnormalities of the second pharyngeal cleft can produce fistulas, sinuses (which may open to the exterior), cysts, and pharyngeal masses.  Some require surgery (Gamble, 1998).  Other abnormalities in development can result in remnants of branchial cartilages that do not degenerate, masses of thymus tissue which remain in the neck or are connected to the parathyroid glands, parathyroid glands that do not descend, and accessory thyroid glands.



The pituitary gland begins its development as a pouch which forms in the roof of the pharynx (Rathke’s pouch).  This tissue must migrate to the site of the developing hypothalamus.  In humans, this migration does not always occur flawlessly.  In some individuals, pituitary tissue remains in the roof of the pharynx as the pharyngeal hypophysis.

 In humans, the thyroid must migrate from its origin in the floor of the pharynx to its destination around the larynx.  As it migrates it remains connected to the tongue to through the thyroglossal duct.  In some people a thyroglossal fistula persists as a remnant of this duct and abherrent thyroid tissue may be found in some individuals along the path of the thyroid from its origin in the mouth to its destination around the larynx (Sadler, p. 312). The parathyroid glands and the thymus must also migrate from their origins to their adult positions (posterior to the thyroid gland for the parathyroid gland, behind the sternum for the thymus).  In some individuals, abnormalities in this migration can result in accessory glandular tissue or tissue located in abnormal positions.  Thymic cysts can produce neck masses in cervical region.  Thymic neck masses in infants may be the remnants of descending thymic tissue or the only site of thymic tissue (Delbrouck, 2002; Loney, 1998; Tovi, 1978).


In the human embryo, the stomach begins its development as a tubular region of the gastrointestinal tract which begins as a generalized tube which then widens and rotates (Sadler, p. 241).  Abnormal rotation of the gut can cause complications in newborns and infants, and rarely can appear in adults as well (Konings-Beetstra, 1990).

      Originally the pancreas is composed of separate dorsal and ventral structures.  When the two duct systems fuse, the dorsal duct may be lost or retained as the accessory duct of the pancreas (Sadler, p. 246-7).  In some individuals, additional pancreatic tissue (heterotopic pancreatic tissue) can be found anywhere from the esophagus to the intestines (Sadler, p. 247).  Some people have a bifid gall bladder (Sadler, p. 245).  The repression of the gene Sonic hedgehog in a restricted region of the posterior foregut causes the pancreas to develop.  Expansion of the area in which Shh is repressed can cause pancreatic tissue to develop in a larger area, including the stomach and duodenum (Kim, 1998). 

     Although human embryos do not require a yolk sac for nutrition (given the existence of the placenta), a yolk sac forms nonetheless and the herniation of the intestines can persist as a serious birth defect.  In 1 in 5,000 births the intestines are herniated while in 1 in 10,000 births, the liver and intestines are herniated (Moore, p.  288).  In the 5th embryonic week the midgut herniates out of the body only to return in the next several weeks, rotating about 270 degrees.  In some cases, the rotation does not reach this degree, leading to abnormalities and potential complications (Konings-Beetstra, 1990).  As the intestines return to the body, they can adhere to other structures, as can the cecum (Moore, p. 290). 

Some people have duplicated portions of their intestines.  In one case there was a duplication of the rectum which contained gastric tissue (Sadler, p. 254).  Many parts of the gastrointestinal tract can be duplicated, especially the ileum.  The duplicated portion may or may not join with the lumen of the GI tract (Burnett, 1953).

In most vertebrates (and even some primitive mammals), the digestive, urinary, and reproductive tracts terminate in a single opening called the cloaca.  Although these tracts are completely distinct in human adults, human embryos form a cloaca which receives these other tracts.  In some infants, the cloaca persists.  In some, there is an abnormal division of the cloaca and the anus may empty into the urethra.  Most anorectal abnormalities result from an unequal division of the embryonic cloaca (Moore, p.  299-300).  The colon may have a double termination and release fecal matter through the vagina (Chaterjee, 1969).
The metanephric kidney must rise cranially during fetal development.  As it ascends, new branches of the aorta must form new renal arteries.  In some people, embryonic renal arteries persist as supernumerary renal arteries. Accessory urinary arteries may exist in 25% of people and may number between 2 and 4 (shell 311-2).
If a kidney does not ascend, it forms a pelvic kidney. If the two ascending kidneys become too close to one another, they may fuse to form a horseshoe kidney which occurs in about 1 in every 600 people (Sadler, p. 267-8).
Two kidneys may also fuse to form a duplex kidney.
duplex kidney


     Some infants possess an ectopic ureter which can open into the neck of the bladder, the urethra, the vagina, or uterus. Extra ureters may form from mesonephric ducts.  As the distance between the two increases, there is an increased probability that the ureter will end blindly or fuse to another embryonic ducts.   In some people there are double ureters, or the ureter may empty into the vagina or urethra.  Some ureters are bifid, although one branch may end blindly.  Instances of uterovaginal duplication are also known (Acien, 1990; Babcock, 1977; Peterson, 1975; Sadler, p.266).

abnormal ureter


.  In the human embryo, the uterus begins as a pair of uterine tubes which begin their fusion caudally (Sadler, p. 282).  The condition known as uterus didelphys occurs if the two female tubes fail to fuse producing a double vagina and uterus.  The condition known as uterus bicornis results from incomplete fusion.  A septum may persist in the uterus and the cervix may be double (Jarcho, Julius).  HOXA13 mutations cause the formation of a bicornuate uterus (Manouvrier-Hanu, 1999). 

uterus abnormal uterus

     Although adult humans do not possess a cloaca, the tissues derived from the embryonic cloacal folds do form part of the reproductive system.  The cloacal folds form the urogenital folds (which form the penis in males and the labia minora in females) and anal folds.  In males with hypospadias, there are abnormal uretral openings because the fusion of the folds was not complete (Sadler, p. 283-4).

ectopic testes
     In cryptorchism, one or both of the testes do not descend into the scrotum in infant males.  Because of the high internal body temperature, a testis maintained within the body will be unable to produce mature sperm. About 30% of premature males and 3-4% of full term males suffer from undescended testes.  Other male infants have ectopic testes which pass through the inguinal canal but do not reach the scrotum.  A testis can even migrate into the penis (Concodora, 1976; (Moore, p.  343). 
extra nipples

Some females are born with an extra ovary (Cruikshank, 1990).

Females may have remnants of the mesonephros which form structures known as the appendix vesiculosa, epoophoron, paraphoron, and duct of Gartner.  In males, persistent mesonephric tissue may form the appendix of the testes (Moore, p.  333). 

     Some fetuses suffer from the absence of the clitoris or penis (Moore, p.  338) or the absence of the vagina (Moore, p.  339).

Embryonic mammals develop a mammary ridge along the ventral trunk.  Different species of mammals develop breasts and nipples at different point along this ridge.  In humans, additional breasts and nipples can develop along this ridge.