The structure of the human body is organized into groups of three with remarkable frequency. I have listed several examples of this tendency by region. This list does not include structures contributing to form the many "triangles" in the body. If you know of any more, please let me know (jmcnulty@bsd.meddean.luc.edu)
Embryology
3 germ layers- endoderm mesoderm ectoderm 3 divisions of somites- sclerotome dermatome myotome 3 definitive kidneys- pronephros mesonephros metanephrosBack
3 parts of the erector spinae m.- iliocostalis longissimus spinalis each are divided again into 3 parts 3 parts of the transversospinalis m. - semispinalis m. multifidus m. rotator m. 3 coverings of the spinal cord- dura mater arachnoid piaNeck
3 parts of the axillary artery 3rd part has 3 branches- subscapular anterior humeral circumflex posterior humeral circumflex Brachial plexus 3 trunks- upper middle lower 3 posterior divisions 3 anterior divisions 3 cords- lateral medial 3 branches off medial cord- medial brachial medial antebrachial medial pectoral posterior 3 branches off posterior cord- upper subscapular lower subscapular thoracodorsal 3 branches off the thyrocervical trunk- transverse cervical suprascapular inferior thyroid 3 veins drain the thyroid gland - superior (into internal jugular) middle (into internal jugular) inferior (into left brachiocephalic) 3 scalene muscles- anterior middle posterior 3 constrictor muscles- superior middle inferior 3 parts of the pharynx- nasopharynx oropharynx laryngeopharynx 3 parts to the hyoid bone- body greater horn lesser hornHead
3 parts of the maxillary artery- mandibular pterygoid pterygopalatine 3 divisions of the trigeminal n.- V1 - ophthalmic V2 - maxillary V3 - mandibular- has 3 parts undivided trunk anterior division posterior division- has 3 branches auriculotemporal lingual inferior alveolar 3 cranial fossae- anterior middle posterior 3 cranial nerves pass through jugular foramen- C.N. IX C.N. X C.N. XI 3 cranial nerves travel in the wall of the cavernous sinus- C.N. III C.N. IV C.N. V 3 cranial nerves carry parasympathetics to the head- C.N. III C.N. VII C.N. IX 3 paired paranasal sinuses- maxillary frontal ethmoidal (sphenoid sinus is not paired) 3 muscles innervated by 3 cranial nerves attach to the styloid process- stylohyoid - C.N. VII stylopharyngeus - C.N. IX styloglossus - C.N. XII 3 layers in tympanic membrane (derived from 3 germ layers) 3 middle ear bones- incus malleus stapes 3 semicircular canals 3 intrinsic muscles of the tongue- longitudinal vertical transverseShoulder
3 muscles attach to coracoid process - coracobrachialis pectoralis minor short head of biceps 3 parts of the axillary a.; 3rd part has 3 branches- subscapular a. anterior circumflex humeral a. posterior circumflex humeral aArm
3 muscles in anterior comparment- biceps brachii coracobrachialis brachialis 3 heads of triceps brachii m.- long lateral medialForearm
3 muscles in deep anterior compartment - flexor digitorum profundus flexor pollicis longs pronator quadratusHand
3 muscles in thenar eminence- opponens pollicis flexor pollicis brevis abductor pollicis brevis 3 muscles in hypothenar compartment- flexor digiti minimi abductor digiti minimi opponens digiti minimi 3 phalanges in fingers- proximal middle distal 3 palmer interosseiThorax
3 compartments- mediastinum left pleural cavity right pleural cavity 3 branches off the aortic arch- brachiocephalic a. left common carotid a. left subclavian a. 3 principal surfaces of the pleura- costal diaphragmatic mediastinal *the cupola is the superior extension 3 lobes of the right lung- superior middle inferior 3 bronchopulmonary segments of the upper right lobe- apical posterior anterior 3 false ribs on each side 3 cusps of the semilunar aortic and pulmonary valves of the heart 3 cusps of the tricuspid valve 3 main arteries of the heart- circumflex anterior interventricular right coronary 3 splanchnic nerves- greater lesser leastAbdomen
3 folds in anterior abdominal wall- median (obliterated urachus) medial (obliterated umbilical a.) lateral (inferior epigastric a.) 3 openings in diaphragm- aortic esophageal inferior vena cava 3 arcuate ligaments of diaphragm- median over aorta medial over psoas major m. lateral over quadratus lumborum m. 3 parts of the stomach- fundus body pylorus 3 unpaired branches of the abdominal aorta- celiac a. superior mesenteric a. inferior mesenteric a. 3 brances of celiac a.- left gastric a. splenic a. common hepatic a. 3 brancheds of common hepatic a.- right gastric a. proper hepatic a. gastroduodenal a. 3 arterial branches supply adrenal glands- inferior phrenic aorta renal 3 structures lie within porta hepatis- proper hepatic a. portal vein common bile duct 3 principal veins contribute to the portal vein- splenic v. superior mesenteric v. inferior mesenteric v. 3 parts of the small intestine- duodenum jejunum ileum 3 parts of large intestine- ascending transverse descending 3 coverings of the spermatic cord- external spermatic fascia cremasteric fascia internal spermatic fasciaPelvis and Perineum
3 bones in os coxae- pubis ilium ischium 3 parts of the pubis- body superior ramus inferior ramus 3 foramina in the pelvis- obturator foramen greater ischiadic foramen lesser ischiadic foramen 3 gluteus muscles- gluteus maximus gluteus medius gluteus minimus 3 rectal arteries- superior rectal from inferior mesenteric a. middle rectal from internal iliac a. inferior rectal from internal pudendal a. 3 branches of the posterior division of internal iliac a.- iliolumbar lateral sacral superior gluteal 3 parts of the urethra- prostatic membranous spongy 3 parts of the uterus- fundus body cervix 3 erectile compartments of the penis- corpus spongiosum left corpus cavernosum right corpus cavernosum 3 muscles in superficial perineal pouch- bulbospongiosus ischiocavernosus superficial transverse perineus 3 arteries supply the rectum- superior rectal (from inferior mesenteric) middle rectal (from internal iliac) inferior rectal (from internal pudendal)Gluteus
3 gluteal muscles- gluteus maximus gluteus medius gluteus minimus 3 muscles innervated by superior gluteal n.- gluteus medius gluteus minimus tensor fascia lataThigh
3 hamstring muscles- biceps femoris semitendinosus semimembranosus 3 compartments in the femoral sheath 3 muscular compartments of the thigh- anterior medial posterior 3 branches of the lateral femoral circumflex a.- descending branch transverse branch ascending branchLeg
3 muscles in anterior compartment - tibialis anterior extensor digitorum extensor hallucisFoot
3 cuneiform bones- medial intermediate lateral 3 phalanges in toes- proximal middle distal 3 plantar interosseiMiscellaneous
3 exams in the course