Below the os hyoides, the anatomy, which involves the surgical relations of the larynx arid trachea, becomes of extreme importance. Be tween the two layers of the fascia superficialis the platysma no longer intervenes ; they ac cordingly lie together and are blended. The vaginal processes of cervical fascia, which have isolated the sub-hyoid muscles, become united into a strong and single raphe along the middle line, from above to within a short distance of the sternal notch ; but here the layers remain distinct, a superficial one fixing itself to the notch and to the interclavicular ligament, while the deeper one descends with the muscles into the medtastinum. The interval contains loose cellular tissue, and sometimes (as Burns no ticed) an absorbent gland. Accordingly, in the very median line, an operator may expose the larynx, trachea, or thyroid body without divi ding or displacing any portion of muscle ; but a lateral deviation from this imaginary line would imply an exposure of the sub-hyoid muscles on one side or on the other. Indeed, the muscles so nearly approach to the line in question, and constitute in their laminar ar rangement so useful a guide to the subjacent parts, that the bare possibility of avoiding them is wisely neglected, and the surgeon teams from them his nearness to the organs which they cover.
In tracing, from the hyoid bone downward, the irregular profile of the air-tube, the finger may distinguish through the integ,ument the following chang,es of outline. 1. A horizontal semicircular notch, limited below by the pro minent angle of the thyroid cartilage, and cor responding, in the interval of the muscles, to the thyro-hyoid membrane ; the lateral parts of this give passage, as we shall presently see, to the laryngeal artery and nerve, biu its mesial part, with which alone we are now occupied, has only a small twig from the thyroid artery ramifying over it: the membrane is thick, and composed of strong vertical fibres in the median line ; it becomes weaker and of laxer tissue in proceeding backward. Its deep aspect contri butes to the skeleton of the pharynx, and cor responds to the epiglottis, from the attached portion of which it is separated only by cel lular tissue and the epiglottidean gland; while, above, the mucous membrane, in being folded forward to the epiglottis, intervenes between it and the membrane. This notch is frequently invaded by the knife of the suicide ; and there is perhaps no part of the neck on which a gash may be inflicted with less serious injury : the large vessels are far removed, and the larynx lies below the blade, which may, if near to the hyoid bone, enter the pharynx above the epi glottidean fold of mucous membrane, leaving the epiglatis unhurt, or, if more nearly ap proached to the thyroid border of the space, may partly or entirely sever that cartilage from its inferior attachments. No special surgical operation belongs to the space ; if indeed we except a proposal made by NI. Malgaigne* for reaching the larynx through it, which has not yet received the sanction of pmctice. 2. The angle,in which the aim of the thyroid car tilage meet, having—under the quaint name of pomum Adami—its extreme prominence above. Within it are the essential organs of voice, which, buckler-like, it protects : the inward aspect of its angle attaches the vocal ligaments; its outward jutting marks their length, and mea sures the development of the larynx. Ilence the pomuna Adana, as indicating by its promi nence that matured growth of the organs of voice, which belongs to male puberty, is a phy siognomical chamcter of sex. Desault's mode of laiyngotomy consists in a vertical division of this angle from below upward, and has the recommendations of easy performance and of efficiency for the extraction of a foreign body. That it invades parts of high functional endow ment and extreme irritability,—that ossification of the cartilage may unexpectedly prevent its completion,—that perfect reunion of the di vided structure is uncertain—are alleged as objections to it, and perhaps over-estimated as such ; for to the fiist may be answered, that the operation is of relief, and hence little likely to aggravate an irritation, the cause of which it removes; to the second may be con ceded,that the mode of operation is not eligible for cases likely to present the bony deposit referred to ; and against the third may be ad duced the evidence of the French surgeons, by whom chiefly the operation has been per formed, that the parts are as quickly repaired, and their functions as completely recovered, as after any other mode of operative procedure. As regards its anatomy, nothing can be easier than to lay bare the pomum Adami; a division of the skin, of the superficial and proper fascize, with some lateral displacement of the sub-hyoid muscles, will suffice for its exposure : and, for its division,—the closest following of the me dial line, in order that the knife may pass be tween the vocal ligaments, leaving both unin jured, is the chief precaution to be observed. The upper edge of the glottis is on a level just below that of the superior thyroid notch. The prominence of the thyroid cartilage and the unyielding support which the borders of its arched surface receive from the bony column behind it, render it liable to be crushed by any considerable, direct, antero-posterior violence. Such has, more than once, been the cause c.f immediate death where a straightforward blow has reached the larynx in prize-fighting ; and such, too, is a not infrequent effect in death by hanging, especially where, as in the English mode of judicial execution, the rope is made to tighten itself jerkingly. The thyroid carti lage is sometimes partially divided in attempts at self-destruction, which it commonly frus trates by defending more important parts. 3.A depression which answers to the crico-thy roid ligament : it is here that the usual opera tion for urgent glottic dyspncea is performed. The common integuments and the fascial raphe cover the little interspace in question, which is safely reached—between the crico-thyroidei— by displacing in a slight extent k,:le sub-hyoid muscles. It has about half an inch of trans verse breadth, and about a third of an inch of height,—is bounded by the inferior thyroid notch and by the (interior part of the circum ference of the cricoid cartilage; which borders give attachment to the strong yellow elastic membrane that closes the space. This depres
sion is so readily felt through the integuments —its boundaries are so definite and its relations so simple, as to render it a peculiarly eligible spot for bronchotomy, when suddenly and ur gently required. A small artery sometimes forms, with its fellow of the opposite side, a transverse communication across this mem brane, and its presence has been much insisted on as a circumstance of practical importance : it is of extreme minuteness, and by no means constantly present : it is the crieo-thyroid, and arises from the thyroid branch of the external carotid, near the upper angle of the thyroid body, and runs across the membrane toward the median line. The necessity for haste is coinmonly of too urgent a character to admit of any deliberate, layer-by-layer, dissective opera tion : a single steady puncture with a canulated trocar, or with a bistoury—directly followed by a tube--is the usual mode of conducting it. In such instances the minute artery can hardly be avoided with certainty, but neither can its division be injurious, since the closely fitting canula will secure the cavity of the air-tube against its trifling, hemorrhage. In the rarer cases, where time is allowed for a slower divi sion of the tissues, it would be desimble not to puncture the membrane till the artery, if pre sent, had been disposed of. It usually lies near to the border of the cricoid cartilage, and might easily be drawn downward away from injury; or its division might be rendered harm less by torsion, or by a fine ligature. In the more extempomneous mode of laryngotomy the bistoury should be guided flatly, close beneath the thyroid cartilage; in so making a transverse division of the membrane, it is parallel to the line of the artery, but above its usual position. 4. The slight prominence of the cricoid car tilag,e, and the series of tracheal rings—be coming progressively deeper toward the ster nurn,—are next felt. In some subjects their chain is seemingly interrupted by a transverse fleshy eminence (which, however, is in health generally imperceptible through the skin), the isthmus of the thyroid gland. To the la teral portions of this body I shall presently return : the isthmus is its only part having re lations in the median line, which it crosses to a very variable extent. Most frequently it measures about half an inch in breadth, and corresponds by its middle to the second ring of the tmchea : but from this, its normal ex tent may vary on the one hand to the ex treme of entire absence—on the other to that of being an uncontracted, flattened union of the lateral lobes, which it may so equal in its vertical dimension. Downward from its lower edge, in front of the remaining rings of the tmchea, passes the inferior thyroid venous plexus, on a level with which would be found, in rare cases, the middle thyroid artery (of Neubauer) ascending from the aortic arch : these vessels are covered by a layer of fascia dividing them from the sterno-thyroid muscles. These parts are variously involved in the two remaining modes of bronchotomy; one of hich—the tracheal—consists in dividing three or four rings of the tube, below the isthmus of the thyroid gland; the other—the crico-tracheal —in dividing its upper rings and with them the cricoid cartilage of the larynx. The tirst—tra cheolonoy—(after a vertical division of the tegumentary parts and a separation of the muscles from the lower part of the larynx to the sternum) exposes the tube in that portion of its extent in which it is deepest and most nearly related to vessels. The operator is required to bear in mind the possible presence of a middle-inferior thyroid artery, lest he wound it inadvertently; he must avoid, or, before opening the air-tube, must secure the inferior thyroid veins ; in recollecting the great lateral mobility of the trachea and its close parallelism to the carotid arteries in the lower part of the neck, he must guard against any oblique glancing of his knife, by which these great vessels might be injured ; in proceeding to divide the cartilaginous rings, he must com mence below and on a completely exposed part of the tube, and with the blunt border of his knife toward the middle line of the sternum, and with its point directed slightly upward, lest (as might happen in neglect of these precautions) the great vena innominata, transversely crossing the tube just below the level of the sternum, or the large arterial trunks, which are there diverging from the median line, should sustain injury: nor must he rudely transfix the tube and encounter the risk of puncturing parts, normally or abnormally be hind it.* The second opemtion, erico.traeheo tomy, first proposed by Boyer,-I- pretends to preference over that just mentioned, on the ground of obtaining,' an equally free opening with less invasion of important parts. Indeed, although AI. Boyer, in proposing it, seems to have considered the section of the thyroid isthmus inevitable, and accordingly included its division in his estimate of risks,—perhaps even that objection might be withdrawn from the operation, if performed in exact agreement with his description ; since the finger tnay de press the thyroid body to an extent which admits a safe division of the first two rings of the tmchea. But it seems to have escaped his notice, while theorising on the operation, that a section of the cricoid cartilage must be use less, unless abused ; that a rigid ring, divided at one point of its circuinference, remains un loosened; that a single section of the cricoid cartilage could not be made available as a means for increased access to the air-tube, over and above that afforded by division of the tra chea,except by em ploying on it a disruptive force, that should effect a counter-fracture at some other part of its circumference. Such violence on such an organ M. Boyer was far too judi cious a surgeon to have sanctioned ; and from the single instance, appended (p.142 bis) to his speculations on the subject, it appears probable that the upward extension of his opening in the air-tube was useless ; that an incision through the upper rings of the trachea sufficed for the escape of the foreign body; and that, in all essential particulars, the crico-tracheal opera tion is but tracheotomy at a higher than ordi nary level, compliatted with an unadvantageous and therefore objectionable intrusion on the larynx.