VASCULAR SYSTEM 1213 



The plica semilunaris.— At the inner cantluis of the palpebral fissure there is 

 a small fold of conjunctival membrane, the plica semilunaris. In many ne«rroes it 

 contains a small plate of cartilage, and it rei)resents the third evelid, or nfctating 

 membrane, of Ijirds, many reptiles, and some amphibians. The function of the 

 third eyelid is to cover and cleanse the front of the eveball, but this function is 

 performed in man by the upjjer eyelid, and consequently the nictating membrane 

 remains as a rudimentary fold. 



VASCULAR SYSTEM 



The foramen ovale. — This aperture is found in the interauricular septum, 

 at the \\\)\)v\- i.art of the fo.ssa ovalis. It is a remnant of a much larger aperture, 

 Avhich existed before birth for the passage of l)lood from the inferior vena cava 

 directly into the left auricle. At birth, when the lungs become functional, the 

 inferior vena cava blood is diverted into the right ventric-le, and in the majority of 

 cases the foramen ovale soon closes, but in one case in five the upjier part remains 

 open in adult life. 



The sulcus terminalis and crista terminalis. — Upon the anterior surface of 

 the riglit auricle there is a shallow sulcus, the svdcus terminalis, which extends 

 from the front of the lower end of the sui)erior vena cava to the right of the termi- 

 nation of the inferior vena cava, and upon the inner surface of the auricular wall, 

 in a similar position, there is a ridge, the crista terminalis, in which the upper 

 ends of the musculi i:»ectinati terminate. Both these structures indicate the line of 

 union of the posterior portion of the primitive heart, the saccus reuniens or sinus 

 venosus, with the second portion or auricle. 



The ligamentum arteriosum. — This term is applied to a fibrous cord which 

 connects the root of the left pulmonary artery to the under surface of the 

 arcli of the aorta, intervening between the superficial cardiac plexus on the inner 

 side and the left recurrent laryngeal nerve on the outer side. It is the remnant of 

 the dorsal part of the left fifth aortic arch, one of a series of vessels arranged in 

 pairs between the ventral and dors^al parts of the aorta of the foetus. AMien the 

 foetal blood-channels are transformed into the permanent vessels of the adult, the 

 ventral parts of the fifth arches become the roots of the pulmonary arteries, the 

 dorsal part on the left side soon disappears, but the dorsal part on the right side 

 persists till birth, and through it blood passes from the right ventricle into the 

 aorta. After birth, Avhen the blood of the right ventricle is diverted into the 

 lungs, the dorsal part of the left fifth arch loses its lumen and is converted into a 

 fibrous cord. 



The obliterated hypogastric arteries. — The filirous cords which are known 

 as the obliterated hypogastric arteries are easily seen when the aVtdomen is opened 

 and the intestines are lifted out of the pelvis. Each cord extends from the anterior 

 part of the upper extremity of the internal iliac artery, under cover of the perito- 

 neum, and beneath the ureter and the vas deferens, or the round ligament of the 

 uterus, according to the sex, to the posterior surface of the anterior wall of the 

 abdomen, where it ascends with an inward inclination across Hessell)ach"s triangle 

 and behind the lower part of the rectus abdominis muscle, to join the urachus a 

 short distance above tlie apex of the l)ladder; both the cords then ascend witli the 

 urachus to the umbilicus, where they terminate in the cicatrix. They are the 

 remnants of the ventral portions of two large arteries, the umbilical arteries, which 

 conveyed blood to the placenta and which formed during the greater i>art oi inter- 

 uterine life the main continuations of the aorta. Tlie dorsal j^ortions of the 

 umbilical arteries become the common and internal iliac arteries, and after birth, 

 when the placental circulation ceases, the ventral parts are converted into fibrous 

 cords. 



The round ligament of the liver.— This ligament is a round, fibrous cord, 

 which extends from the umbilical cicatrix along the lower or free border of the 



