Loyola University Medical Education Network List of Three's in Anatomy

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 John A. McNulty, Ph.D.


3 body planes - frontal sagittal transverse 3 layers of skin- epidermis dermis hypodermis 3 general venous circulations- systemic pulmonary portal


3 germ layers- endoderm mesoderm ectoderm 3 divisions of somites- sclerotome dermatome myotome 3 definitive kidneys- pronephros mesonephros metanephros 3 derivatives of skull- desmo/dermatocranium splanchno/viscerocranium chondrocranium 3 initial embryonic divisions of the brain- prosencephalon mesencephalon rhombencephalon


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 pia 3 spaces surrounding spinal cord- epidural subdural subarachnoid 3 borders of the scapula- medial (vertebral) lateral superior 3 angles of the scapula


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 muscles attach to spine of C2- semispinalis cervicis obliquus capitis inferior rectus capitis posterior major 3 constrictor muscles- superior middle inferior 3 parts of the pharynx- nasopharynx oropharynx laryngeopharynx 3 parts to the hyoid bone- body greater horn lesser horn 3 structures inside carotid sheath- common carotid artery internal jugular vein vagus nerve 3 ganglia in sympathetic cervical chain- superior (magnum) middle inferior (stellate)


3 parts of the maxillary artery- mandibular pterygoid pterygopalatine 3 divisions of the trigeminal n.- V1 - ophthalmic - has 3 branches frontal nasociliary lacrimal 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 nuchal lines- inferior superior highest 3 layers to calvarium- outer table diploe inner table 3 clinoid processes of the sphenoid bone- 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 nasal conchae- superior middle inferior 3 principal components of the temporal bone- squama petro/mastoid tympanic ring 3 bones of the upper jaw/palate- premaxilla maxilla palatine 3 salivary glands- parotid submaxillary sublingual 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 cranial nerves innervate the external tympanum- C.N. V C.N. VII C.N. X 3 middle ear bones- incus malleus stapes 3 parts of the ear- outer middle inner 3 semicircular canals 3 membranes of the cochlea- Reissner Tectorial Basilar 3 intrinsic muscles of the tongue- longitudinal vertical transverse 3 muscles elevate the mandible- masseter m. temporalis m. medial pterygoid m. 3 branches of the posterior auricular artery- stylomastoid branch auricular branch occipital branch 3 branches of the ophthalmic nerve (VI)- nasociliary n. frontal n. lacrimal n. 3 superior alveolar nerves- anterior superior alveolar n. middle superior alveolar n. posterior superior alveolar n. 3 types of deciduous teeth- incisors canine molars 3 layers of dentition- enamal dentin pulp 3 general layers of the eye - retinal choroidal/ciliary body scleral/corneal 3 color receptors in cone cells - red blue yellow 3 nerves to the larynx - external laryngeal n. internal laryngeal n. recurrent laryngeal n. 3 cerebral arteries - anterior middle posterior 3 arteries supply the cerebellum - superior cerebellar anterior inferior cerebellar posterior inferior cerebellar


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 a 3 muscles of the rotator cuff are rotators 3 glenohumeral ligaments - superior middle inferior


3 muscles in anterior comparment- biceps brachii coracobrachialis brachialis 3 heads of triceps brachii m.- long lateral medial 3 muscles attach to greater tubercle of the humerus- supraspinatus infraspinatus teres minor 3 parts of the ulnar collateral ligament of the elbow - anterior posterior oblique


3 muscles in deep anterior compartment - flexor digitorum profundus flexor pollicis longs pronator quadratus


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 interossei


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 parts of the aorta- ascending arch descending 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 muscles of interthoracic wall- external intercostal internal intercostal intercostalis intimus 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 least


3 folds in anterior abdominal wall- median (obliterated urachus) medial (obliterated umbilical a.) lateral (inferior epigastric a.) 3 muscles contribute to linea semilunaris/rectus abdominus aponeurosis - internal oblique external oblique transversus abdominus 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 smooth muscle layers of the stomach- oblique longitudinal circular 3 unpaired branches of the abdominal aorta- celiac a. superior mesenteric a. inferior mesenteric a. 3 branches of celiac a.- left gastric a. splenic a. common hepatic a. 3 branches 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 tenia of large intestine 3 coverings of the spermatic cord- external spermatic fascia cremasteric fascia internal spermatic fascia 3 vessels in the umbilical cord umbilical vein 2 umbilical arteries

Pelvis 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 openings in the urinary bladder- left ureter right ureter urethra 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) 3 valves in the rectum- superior middle inferior


3 gluteal muscles- gluteus maximus gluteus medius gluteus minimus 3 muscles innervated by superior gluteal n.- gluteus medius gluteus minimus tensor fascia lata


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 branch 3 muscles contribute to the pes anserinus- sartorius semitendinosus gracilis


3 compartments in the leg 3 muscles in anterior compartment - tibialis anterior extensor digitorum extensor hallucis 3 branches of the peroneal artery in the leg - anterior tibial a. posterior tibial a. peroneal a. 3 peroneal muscles in the leg - peroneus longus m. peroneus brevis m. peroneus tertius m. 3 muscles that comprise the triceps surae - grastrocnemius m. soleus m. plantaris m. 3 muscles in the deep posterior compartment - tibialis posterioris m. flexor hallucis longus m. flexor digitorum longus m.


3 cuneiform bones- medial intermediate lateral 3 phalanges in toes- proximal middle distal 3 plantar interossei 3 described arches - medial lateral transverse


3 exams in the course

If you like this kind of stuff, you may want to visit Michael Eck's Book of Threes

Other contributors include:

    Lora Leuzinger
  1. Diego Villa Restrepo
  2. Cindy Kuiper
  3. Erika Goodwin
  4. Rob Grieve


Please contact: John A. McNulty, Ph.D.
Last Updated: Feb 21, 2006
Created: May 1, 1994