Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
Varieties occasionally observable in the verte¬ 
bral arteries. — ]. Of origin.—a. It has already 
been mentioned that the vertebral artery 
may arise from different portions of the first 
stage of the subclavian artery being sometimes 
nearer, and sometimes further removed, from 
the innominata; but, independently of these 
varieties, the vertebral artery on the right 
side is 
b. Sometimes furnished by the common 
carotid artery. In all the cases where this 
anomaly has been observed, the right subcla¬ 
vian artery was given off directly as a branch 
of the aorta. Again, 
c. The vertebral artery sometimes comes 
off from the arch of the aorta. This irre¬ 
gularity is as unfrequent on the right, as it is 
common on the left side. When the left 
vertebral artery springs from the arch of the 
aorta, it usually arises between the left carotid 
and the left subclavian arteries, though some¬ 
times its origin has been found to the left of 
all the other branches of the arch. 
d. In a few instances, more than one vessel 
has been found to constitute the origin of the 
vertebral artery; thus, it may be formed by 
the union of two roots, both arising from the 
subclavian artery, or one from the subclavian 
and the other from the aorta. In one exam¬ 
ple, where it was formed by three roots, two of 
these were derived from the subclavian, and 
the third from the inferior thyroid artery. 
These roots of the vertebral artery in some 
instances united before the artery had become 
engaged in the vertebral foramina, whilst in 
others the union took place subsequently. 
2. Of size.—There is often a considerable 
difference in the size of the two vertebral arte¬ 
ries, which is stated to be most frequently in 
favour of that of the left side ; thus, in 98 ob¬ 
servations made by Mr. Davy, the left verte¬ 
bral artery was the larger in twenty-six, and 
the right in eight instances only. 
3. Of course and relations.—The vertebral 
artery may enter the transverse process of the 
last cervical vertebra (though the contrary has 
been asserted), or it may enter one of the fo¬ 
ramina higher than that in the transverse pro¬ 
cess of the sixth, which latter it usually selects. 
When the artery enters any vertebra higher 
than the sixth cervical, it always occupies an 
unusually superficial position in the neck, 
lying external and parallel to the common 
carotid artery, for which, consequently, it is 
liable to be mistaken. ( Vide Carotid Ar¬ 
The vertebral vein corresponds to the cer¬ 
vical stage only of the artery. Its origin is 
found in some branches from the deep muscles 
at the back of the neck, joined by one from 
the occipital vein, and by another which 
passes through the posterior condyloid fo¬ 
ramen. The vertebral vein traverses the 
canal in the transverse process of the atlas, 
and descends through the same foramina by 
which the artery ascends ; whilst here it lies in 
front of the artery, and has the same relations 
as that vessel. Emerging from the foramen in 
the sixth vertebra, the vertebral vein (liable 
to the varieties already specified) opens into 
the vena innominata close to its junction with 
the internal jugular. 
II. Internal mammary artery.—This ar¬ 
tery arises (more externally than the ver¬ 
tebral) from the anterior surface of the sub¬ 
clavian, and near to the inner margin of the 
anterior scalenus muscle. From this origin it 
runs, first forwards, then downwards and 
inwards, and enters the thorax, lying between 
the pleura and the internal layer of intercostal 
Previous to entering the thorax, the in¬ 
ternal mammary artery is crossed anteriorly 
by the vena innominata and by the phrenic 
nerve : the latter intersects the artery ob¬ 
liquely from above and without, downwards 
and inwards. In the thorax, however, the 
nerve attains a position much posterior to the 
The mammary artery descends on the back 
of the anterior parieties of the chest a little 
external to the junction of the costal cartilages 
with the sternum, and is covered posteriorly 
by the pleura at the line of reflexion of that 
membrane to form the side of the anterior 
mediastinum. Arrived at the cartilage of 
the third rib, the artery in its farther 
descent inclines a little outwards, and be¬ 
comes separated from the pleura by the 
fibres of the triangularis sterni; the vessel is 
now placed between that muscle, which lies 
behind it, and the internal layer of intercostal 
muscles and the cartilages of the lower true 
ribs, which constitute its anterior relations. 
Opposite the cartilage of the seventh rib the 
mammary artery terminates by dividing into 
two branches, an external and an internal. 
Branches of the mammary artery. — I. 
Mediastinal branches, which are distributed 
to the thymus gland (thymic arteries), and 
the cellular issue of the anterior mediasti¬ 
2. A descending muscular branch to the 
diaphragm (superior phrenic), also termed, from 
its so constantly accompanying the phrenic 
nerve, the comes nervi phrenici. This branch 
accompanies the nerve in a tortuous manner, 
between the pleura and the pericardium, to 
reach the upper surface of the diaphragm, 
where it anastomoses with the phrenic arte¬ 
ries from the aorta. 
3. The anterior intercostal arteries.—These 
are distributed to the six upper intercostal 
spaces ; but their number is greater than that 
of the spaces for which they are destined, as 
two branches are frequently found between 
adjacent ribs, and this arrangement may even 
prevail in all of these intercostal spaces. 
The anterior intercostal arteries pass out¬ 
wards in the intervals between the two planes 
of intercostals, in which muscles some of their 
branches terminate ; others are lost in anasto¬ 
mosing with the intercostal arteries from the 
aorta, whilst many perforate the muscular 
fibres, and, arriving on the external surface 
of the thorax, dip into the pectoral muscles


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