638 THE BLOOD-VASCULAR SYSTEM 



Itj'may not bifurcate at all, in which case the branches usually arising from the external car- 

 otid are derived from the common. The ascending pharyngeal and superior thyreoid occa- 

 sionally arise from an otherwise normal common carotid. Unusual origin of the common 

 carotids has been referred to (see Aorta). 



Branches of the carotid arteries. — The superior thyreoid, lingual and external maxillary 

 sometimes have a common stem of origin. The superior thyreoid artery varies in size inversely 

 with the inferior. The external maxillary occasionally terminates in its submental branch. 

 In such cases the main supply of the face is taken over by an abnormally large dorsal nasal 

 branch of the ophthalmic, or transverse facial branch of the temporal artery. The occipital 

 sometimes arises from the internal carotid or from the ascending cervical. The ascending 

 •pharyngeal is very variable in its place of origin from the external carotid, it may arise from the 

 common or internal. 



Out of 447 arteries examined, the second portion of the internal maxillary passed lateral to 

 the external pterygoid muscle in 55 per cent., and medial to it in 45 per cent, of cases. When 

 medial to this muscle the internal maxillary sometimes passes medial to the inferior alveolar 

 and Ungual nerves and occasionally between them. The variability in the course of this artery 

 appears to depend on a tendency to reduplication of the infraorbital branch of the stapedial 

 artery (p. 634) in the neighbourhood of the mandibular nerve. Such a condition was found 

 by Thyng in a 17 mm. human embryo. When the internal maxillary passes medial to the ex- 

 ternal pterygoid there is often a parallel anastomosing channel between the posterior deep 

 temporal and buccal branches. 



The ophthalmic artery may arise, wholly or in part, from the middle meningeal, or vice versa. 

 This is due to the anastomosis between the supraorbital branch of the stapedial and the oph- 

 thalmic in the embryo. 



Subclavian artery. — Irregularities of origin have been referred to (see Aorta). 



The branches of the subclavian artery are very variable in their place of origin (p. 559). 

 The vertebral may arise directly from the arch of the aorta (p. 537) or take an unusual course 

 in the neck. It may enter the foramen transversarium of the fourth or fifth cervical vertebra 

 instead of the sixth; this arises from substitution of an embryonic precostal anastomosis in these 

 segments for the usual postcostal. By a converse substitution it may enter the seventh. The 

 aa. transversa colli and scapulce vary inversely in size. The arteria aberrans connecting the right 

 subclavian with the dorsal aorta has been referred to (p. 634). 



The thoracic aorta. — Transposition, and the arteria aberrans have been referred to above. 



Branches of the thoracic aorta. — The intercostal arteries are Uable to numerical variation, 

 evidently owing to the occurrence of precostal intersegmental anastomoses between the embry- 

 onic dorsal segmentals. A common longitudinal stem may even take over the vessels of both 

 sides. The anterior spinal artery usually shows lack of median symmetry which indicates the 

 bilaterahty of its origin (p. 636). The arrangement of the bronchial arteries is liable to much 

 variation; this has not received adequate explanation. 



The abdominal aorta sometimes divides as low as the fifth lumbar vertebra, occasionally 

 higher than usual, depending upon the definitive position taken by the umbilical arteries (p. 

 637). Cases are on record of accessory pulmonarj' arteries arising by a single stem from the 

 abdommal aorta, which passes into the thorax along the oesophagus. The aorta and vena cava 

 inferior may be transposed either as a part of situs inversus or as an abnormality of the venous 

 system. 



Branches of the abdominal aorta. — The lumbar arteries are subject to the same type of 

 variation as occurs in the intercostals. There may be a loop connecting the coeliac and superior 

 mesenteric arteries. Any or all of the branches of the coeliac may arise from the superior 

 mesenteric (ca'lio-mesenteric in the latter case) or dkectly from the aorta. The instabiUty of 

 the coeliac and superior mesenteric branches is favored by the rapid cranio-caudal migration 

 of the two trunks; intersegmental anastomosis, in some cases, may be a factor also. There is 

 very great variation in the number of branches given off by the superior mesenteric and in the 

 details of their arrangement. This is a natural result of the number of possible routes which 

 may be taken by the blood; these resemble, in their variety, those of an embryonic circulation. 

 The region of supply of the inferior mesenteric artery is some'times taken over entirely or in part 

 (e. g., middle colic) by the superior mesenteric. An omphalo-mesenteric artery, in rare cases, 

 arises from the superior mesenteric or one of its branches. It passes to the navel and anasto- 

 moses with inferior epigastric and with the small arteries accompanying the round ligament of 

 the liver or the urachus. 



Accessory renal arteries are very common; as many as six have been recorded. These 

 may arise from the aorta, middle sacral, inferior phrenic, middle suprarenal or internal spermatic. 

 According to Felix, these are to be regarded as persistent mesonephric arteries. Those arising 

 above the regular renal frequently enter the kidney dorsal to the hilum. Those below it are 

 more apt to be ventrally placed. 



Nearly all possible varieties of origin are met with in the inferior phrenic, middle supra- 

 renal, internal spermatic and accessory renal arteries which find explanation in the caudal migra- 

 tion of, and anastomosis between, the embryonic representatives of these vessels. The oc- 

 casional origin of the inferior phrenic from the ca'liac (or its branches) or from the superior 

 mesenteric; of the internal spermatic or the middle suprarenal from the lumbar arteries, or 

 of an accessory renal from the inferior mesenteric must be taken as indicating embryonic 

 anastomoses between the dorsal, lateral, or ventral segmental arteries, as the case may be. 



The iliac and hypogastric arteries. — ^'I'he length of the common i/tac depends upon the site of 

 aortic bifurcation (p. 5'.)()); also upon the site of division of the common iliac into external iliac 

 and hypogastric. If these spring directly from the aorta (as they do in rare cases) the common 

 iliac is absent. The trunk formed by the common iliac and hypogastric is the proximal portion 

 of the embryonic umbilioal iirtory. The manner in which this takes over a dorsal segmental 

 artery (probably the fifth lumbar) to become the external iliac is not sufficiently imdersood to 

 account for variations in this region. 



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