ANATOMY OF A 17.8 MM. HUMAN EMBRYO 73 



The left internal carotid (A. card.) comprises the third aortic 

 arch, the left dorso-lateral aorta cephalad of the third arch, 

 and a terminal portion or branch from the first aortic arch. It 

 curves at first dorsally and cephalad, and then extends directly 

 cephalad, dorsal to the roof of the pharynx. It passes medial to 

 the cranial nerves and bifurcates, lateral to the diencephalon 

 (Dien.) into caudal and cephalic divisions. The former is the 

 posterior communicating artery (A.com.p.). This extending 

 caudad gives off the posterior chorioidal artery, and joins the left 

 posterior cerebral (A.cer.p.). The other terminal division of the 

 internal carotid artery divides into two branches, the anterior 

 chorioidal (A.chr.a.), and a stem common to the middle cerebral 

 (A.cer.m.) and the anterior cerebral (Axer.a.) arteries. Besides 

 the terminal branches two other branches of the left internal carot- 

 id are shown here, the ophthalmic and the stapedial. The oph- 

 thalmic artery (A.oph.) arises from the internal carotid artery 

 medial to the ophthalmic nerve (N.oph.). It extends cephalad 

 and laterally, ventral to the optic stalk which it penetrates, be- 

 coming the central artery of the retina. 



The stapedial artery (A.stp., pi. 2) arises from the internal 

 carotid near the middle of the pharynx, and passing through the 

 left stapedial cartilage, runs as shown in plate 2, towards the semi- 

 lunar ganglion (G.s.-L). A short distance from the ganglion it 

 divides into two branches, dorsal and ventral. The dorsal or 

 supraorbital branch passes lateral to the semilunar ganglion into 

 the region of the orbit. The ventral division soon bifurcates, 

 and the resulting branches pass to the medial and lateral sides, 

 respectively, of the mandibular nerve. The medial branch, 

 having communicated with the lateral by a branch passing ven- 

 tral to the mandibular nerve, first accompanies the buccal branch 

 of the mandibular and later the maxillary nerve, as the infra- 

 orbital artery. The lateral branch passes between the roots of 

 origin of the auriculo-temporal nerve and,' having communicated 

 with the medial branch, becomes the inferior alveolar which ac- 

 companies the nerve of the same name (N. ah. inf.). 



The development and importance of the stapedial artery have 

 been demonstrated admirably by Tandler ('02). He finds that 



