THYROID GLAND. (French, glande thyroide ; German, Die Schilddruse ; Italian, Tiroidea glandola ; Latin, Glandula thyroidcea.) The organ which has received this name is a bilobed glandular body, situated in the human subject in close proximity to the larynx, from the prominent cartilage of which it has pro bably derived its appellation.
Its size varies considerably in different in dividuals, according to unknown peculiarities. In the female it is generally larger than in man ; reversing thus the proportion which obtains between the vocal apparatus in the two sexes, and so far negativing the idea, that the larynx and the thyroid gland are in any wise intimately connected.
The normal weight of the thyroid is about one ounce, according to Cruveilhier. Any great excess above this must be regarded as indicating a pathological condition.
Its form, as has been said, is bilobed ; the lateral lobes being united by a thinner and narrower portion termed the isthmus. It would appear (from the circumstance that variations of form are most frequent in the isthmus, which sometimes is even wanting) that the lateral lobes are the primary parts of the gland, and, in fact, in one entire class (that of birds) the lobes lie entirely separate, one on each side of the trachea. In the human subject they are large and solid, presenting an anterior convex surface, a posterior concave, an external and inferior border which is con vex, and runs up to join the superior interior concave border in a pointed cornu, which reaches as far as the origin of the inferior con strictor pharyngis from the ala of the thyroid cartilage. From the upper border of the isthmus, or from the adjacent part of one of the lateral lobes, there stretches upwards a narrow strip of glandular tissue, which has been called the pyramid or mesian column, and which may have been sometimes mis taken for a muscle, as Cruveilhier asserts ; though it is distinguished perfectly from the so called lcvator gland. thyroid. by Haller. This prolongation sometimes extends to the hyoid bone, but generally not so far, and is subject to numerous modifications of shape and structure. The following list of the various forms which the thyroid may pre sent, is taken from the catalogue of the mu seum of Guy's Hospital, which Dr. Birkett, the curator, most kindly allowed me to inspect, together with the preparations. 1. Thyroid without an isthmus, but having two mesian columns. 2. Thyroid almost without an isthmus, and having one mesian column. 3. Thyroid with broad isthmus and one mesian column. 4. Thyroid with no isthmus, but having one large mesian column. 5. Thyroid without isthmus, having one column (right), and an isolated portion on the left. 6. Thy roid with one lobe only developed, and a little glandular body below the middle of thyroid cartilage. 7. Thyroid gland replaced by a membranous substance, two small por tions only remaining. 8. The gland with isthmus and two mesian columns.
It is important to ascertain accurately the situations and relations of the thyroid. I give them as they are stated by Cruveilhier, and confirmed by my own observation. The isthmus lies across the first four rings of the trachea, the first not being completely co vered.* Its upper margin is about half an inch
below the inferior border of the cricoid carti lage : from it the mesian column passes up ward, lying upon the crico-thyroid muscle, the thyroid cartilage, and the thyro-hyoid membrane. The lower border of the isthmus is free, and occasionally descends so low, that there is not space between it and the sternum to perform the operation of tracheotomy. Posteriorly, the isthmus is firmly attached to the rings of the trachea by close and dense areolar tissue. The sterno-hyoid and sterno thyroid muscles overlap the greater part of the isthmus ; a small portion, however, in the median line is covered only by deep cervical fascia, and perhaps crossed by some branches of origin or communication of the anterior jugular veins. The lateral lobes, concave posteriorly, embrace, and rest against the sides of, the trachea, the cricoid cartilage, the in ferior and lateral parts of the thyroid car tilage, and the lower part of the pharynx and upper part of the cesophagus. " These lobes form, with the connecting isthmus, a half or sometimes three fourths of a canal, which surrounds all these parts. This re lation, one of great importance, explains hoW certain goitres flatten the trachea late rally, hinder deglutition, and finally bring on a real asphyxia from strangulation."- The posterior border of the lateral lobes corre sponds to the vertebral column, and rests upon the carotid artery ; but, if enlarged, it extends further outward, and lies upon the jugular vein. Both recurrent nerves ascend behind the lateral lobes, and are closely in relation with them as they pass under the lower edge of the inferior constrictor muscle. Anteriorly, the sterno- and omo-hyoid muscles pass in front of the lateral lobes, the sterno thyroid is stretched as a thin band over their surface, and in cases of considerable hyper trophy may be seen sunk in a deep groove formed in their substance, or, as Cruveilhier states, expanded to a width double or treble its natural size. These muscles separate the thyroideal lobes from the sterno-cleido-mas told, which, with the mastoid artery and the superficialis descendens on its inner surface, overlaps their greater extent, as it passes backwards and upwards to its cranial attach ment. " The superior extremity of each of the lateral lobes terminating in a point, whence the bicorned form which has been attributed to the thyroid body, corresponds on the inside of the carotid artery to the lateral and posterior part of the thyroid car tilage, and extends sometimes even to the neighbourhood of its upper border. The inferior extremity, thick and rounded, descends more or less low in different subjects, and corresponds to the fifth, sixth, or even to the seventh ring of the trachea; it is situated between the trachea and the common carotid artery. By the inferior extremity the inferior thyroid artery reaches the gland." " The superior border is concave, and skirted by the superior thyroid arteries." The inferior is convex, and has branches of the inferior thyroid running along it.