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22 COMMENTARY ON PLATES 7 & 8.
The normal relative anatomy of the bloodvessels is taken by anatomists to be the more frequent disposition of their main trunks and branches, considered per se, and in connexion with neighbouring parts. But it will be seen by this avowal that those vessels are liable to many various conditions; and such is the case, in fact. No anatomist can pronounce with exactness the precise figure of vessels or other organs while they lie concealed beneath the surface. An approach to truth is all that the best experience can boast of. The form and relations of the carotid vessels of Plate 7 may or may not be the same as those concealed beneath the same region of Plate 8, at the point R.
The motions of the head upon the neck, or of the neck upon the trunk, will influence the relative position of the vessels A C B, of Plate 7, and therefore we take a fixed surgical position, in the expectation of finding that the carotid artery projects from under the anterior border of the upper third of the sterno-mastoid muscle, opposite the upper border of the thyroid cartilage; at this situation of the vessels, viz., R, Plate 8, opposite O, the thyroid projection, is in general to be found the anatomical relation of the vessels as they appear dissected in Plate 7. Of these vessels, the main trunks are less liable to anomalous character than the minor branches.
The relative position of the subclavian artery is as liable to be influenced by the motions of the clavicle on the sternum, as that of the carotid is by the motions of the lower jaw-bone on the skull, or by the larynx, in its own motions at the fore-part of the neck. It becomes as necessary, therefore, in the performance of surgical operations upon the subclavian artery, to fix the clavicle by depressing it, as in Plate 8, as it is to give fixity to the lower maxilla and larynx, in the position of Plate 7, when the carotid is the subject of operation.
The same named structures, but different as to their parts, will be found to overlie the subclavian artery as are found to conceal the carotid artery. The skin, the fascia, and platysma muscle, the sterno-cleido-mastoid muscle, the deep layer of the cervical fascia, &c., cover both vessels. One additional muscle binds down the subclavian artery, viz., the scalenus anticus. The omo-hyoid relates to both vessels, the anterior division to the carotid, the posterior to the subclavian.
The carotid artery lies uncovered by the sterno-mastoid muscle, opposite to the upper border of the thyroid cartilage, or the hyoid bone; and the subclavian artery emerges from under cover of a different part of the same muscle, opposite the middle of the clavicle. These points of relationship to the skeletal parts can be ascertained by the touch, in both instances, even in the undissected body. The thyroid point, O, of Plate 8, indicates the line, R N, which the carotid artery traverses in the same figure, along the anterior border of the sterno-mastoid muscle, as seen in the dissected region of Plate 7. The mid-point of the clavicle, U, Plate 7, and the top of the sternum in the same figure, will, while the eye follows the arching line, Z X T V, indicate with correctness the arching course of the subclavian, such as is represented in the dissection of that vessel, B, Plate 8.
The subclavian artery has no special sheath, properly so called; but the deep layer of the cervical fascia, P, Plate 8, which passes under A, the clavicular portion of the sterno-mastoid muscle, and becomes of considerable thickness and density, sheaths over the vessel in this region of its course.
A very complex condition of the veins which join the external jugular at this part of the course of the subclavian artery is now and then to be found overlying that vessel. If the hemorrhage consequent upon the opening of these veins, or that of the external jugular, be so profuse as to impede the operation of ligaturing the subclavian artery, it may in some measure be arrested by compressing them against the resisting parts adjacent, when the operator, feeling for D, the scalenus muscle, and the first rib to which it is attached, cannot fail to alight upon the main artery itself, B, Plate 8.
COMMENTARY ON PLATES 7 & 8. 23
The middle of the shaft of the clavicle is a much safer guide to the vessel than are the muscles which contribute to form this posterior triangle of the neck, in which the subclavian vessel is located. The form or position of the clavicle in the depressed condition of the shoulder, as seen in Plate 8, is invariable; whereas that of the trapezius and sterno-mastoid muscles is inconstant, these muscles being found to stand at unequal intervals from each other in several bodies. The space between the insertions of both these muscles is indefinite, and may vary in degrees of width from the whole length of the clavicle to half an inch; or, as in some instances, leaving no interval whatever. The position of the omo-hyoid muscle will not be accounted a sure guide to the locality of the subclavian artery, since, in fact, it varies considerably as to its relationship with that vessel. The tense cords of the brachial plexus of nerves, F, Plate 8, which will be found, for the most part, ranging along the acromial border of the artery, are a much surer guide to the vessel.
On comparing the subclavian artery, at B, Plate 8, with the common carotid artery, at A, Plate 7, I believe that the former will be found to exhibit, on the whole a greater constancy in respect to the following-mentioned condition--viz., a single main arterial trunk arches over the first rib to pass beneath the middle of the clavicle, while the carotid artery opposite the thyroid piece of the larynx is by no means constantly single as a common carotid trunk. The place of division of the common carotid is not definite, and, therefore, the precise situation in the upper two-thirds of the neck, where it may present as a single main vessel, cannot be predicted with certainty in the undissected body. There is no other main artery of the body more liable to variation than that known as external carotid. It is subject to as many changes of character in respect to the place of its branching from the common carotid, and also in regard to the number of its own branches, as any of the lesser arteries of the system. It is but as an aggregate of the branches of that main arterial trunk which ranges from the carotid foramen of the temporal bone to the aorta; and, as a branch of a larger vessel, it is, therefore, liable to spring from various places of the principal trunk, just as we find to be the case with all the other minor branches of the larger arteries. Its name, external carotid, is as unfittingly applied to it, in comparison with the vessel from which it springs, as the name external subclavian would be if applied to the thyroid axis of the larger subclavian vessel. The nomenclature of surgical anatomy does not, however, court a philosophical inquiry into that propriety of speech which comparative science demands, nor is it supposed to be necessary in a practical point of view.
It will, however, sound more euphoneously with reason, and at the same time, I believe, be found not altogether unrelated to the useful, if, when such conditions as the "anomalies of form" present themselves, we can advance an interpretation of the same, in addition to the dry record of them as isolated facts. Comparative anatomy, which alone can furnish these interpretations, will therefore prove to be no alien to the practical, while it may lend explanation to those bizarreries which impede the way of the anthropotomist. All the anomalies of form, both as regards the vascular, the muscular, and the osseous systems of the human body, are analyzed by comparison through the animal series. Numerous cases are on record of the subclavian artery being found complicated with supernumerary ribs jutting from the 5th, 6th, or 7th cervical vertebrae. [Footnote] To these I shall add another, in respect of the carotid arteries--viz., that I have found them complicated with an osseous shaft of bone, taking place of the stylo-hyoid ligament, a condition which obtains permanently in the ruminant and other classes of mammals.
[Footnote: I have given an explanation of these facts in my work on Comparative Osteology and the Archetype Skeleton, to which, and also to Professor Owen's work, entitled Homologies of the Vertebrate Skeleton, I refer the reader.]
DESCRIPTION OF PLATES 7 & 8.
PLATE 7.
A. Common carotid at its place of division.
B. External carotid.
C. Internal carotid, with the descending branch of the ninth nerve lying on it.
D. Facial vein entering the internal jugular vein.
E. Sterno-mastoid muscle, covered by
F. Part of the platysma muscle.
G. External jugular vein.
H. Parotid gland, sheathed over by the cervical fascia.
I. Facial vein and artery seen beneath the facial fibres of the platysma.
K. Submaxillary salivary gland.
L. Upper part of the platysma muscle cut.
M. Cervical fascia cut.
N. Sterno-hyoid muscle.
O. Omo-hyoid muscle.
P. Sterno-thyroid muscle.
Q. Fascia proper of the vessels.
R. Layer of the cervical fascia beneath the sterno-mastoid muscle.
S. Portion of the same fascia.
T. External jugular vein injected beneath the skin.
U. Clavicle at the mid-point, where the subclavian artery passes beneath it.
V. Locality of the subclavian artery in the third part of its course.
W. Prominence of the trapezius muscle.
X. Prominence of the clavicular portion of the sterno-cleido-mastoid muscle.
Y. Place indicating the interval between the clavicular and sternal insertions of sterno-cleido-mastoid muscle.
Z. Projection of the sternal portion of the sterno-cleido-mastoid muscle.
Plate 7
PLATE 8.
A. Clavicular attachment of the sterno-mastoid muscle lying over the internal jugular vein, &c.
B. Subclavian artery in the third part of its course.
C. Vein formed by the union of external jugular, scapular, and other veins.
D. Scalenus anticus muscle stretching over the artery, and separating it from the internal jugular vein.
E. Post-half of omo-hyoid muscle.
F. Inner branches of the brachial plexus of nerves.
G. Clavicular portion of trapezius muscle.
H. Transversalis colli artery.