The capillary plexus, in which the minute branches of these arteries terminate, is dis posed in the form of hollow spheres around the glandular cavities, closely applied upon the limitary membrane and forming a continuous network throughout the gland. It is tolera bly close-meshed, but not nearly so much so as that of the liver or kidney. The diameter of the capillaries, in a recent injected specimen, varied from inch, and the interspaces were, I think, two or three times as large.
There are corresponding vence comites to the superior and inferior thyroideal arteries. The superior thyroid or thyro-laryngeal returns its blood either into the internal jugular vein, or into the common trunk of the facial and lingual, before it joins either of the jugulars. The middle thyroideal runs down and turns aside, crossing the common carotid to enter the internal jugular. Besides these there exist constantly another pair of veins, which run down in front of the trachea involved in the deepest layer of cervical fascia, and terminate either by opening both in to the vena transversa, or the left into this trunk, and the right into the junction of it with the right brachio-cephalic. These veins run down, gradually diverging from each other ; so that, from being at their origin no more than one third of an inch apart, they are separated at the lower part of their course by an interval of about an inch, or rather more. In this situation they are often united by a transverse branch, and are said, by Cruveilhier, to form, with the tracheal and laryngeal veins proceeding to unite with them, a considerable plexus, which it is impossible to avoid in the operation of tracheotomy. These veins correspond in some measure with the thyroideal artery of Venbauer, but are much more constant, and are sometimes three or four in number; so that the blood they re turn is not proportioned to that conveyed by the artery.
The lymphatics, originating probably in a closed network, proceed to enter the deep cervical glands. They may sometimes be seen filled with a concrete albuminous substance, which they have probably taken up from the glandular cavities.
Nerves.— The recurrent laryngeal, shortly before it passes under the margin of the in ferior constrictor muscle of the pharynx, gives off some filaments to the thyroid gland; some are also furnished by the external laryngeal ; while a plexus, derived from the middle cervical ganglion, proceeds along the inferior thyroid artery, and is distributed to the gland along with its branches, forming communications with the preceding. In thin sections of the thyroid treated with acetic acid, I have seen the nucleated bands of the sympathetic, con taining one or two cmlio-spinal tubules, running fur some distance in the interspaces of the vesicles ; they probably terminate by forming a looping plexus ; but I have not been able to ascertain anything certain on this head respecting either these or the tubular fibres.
Development. — The thyroid is said by Cruveilhier to be developed by two lateral halves, which are subsequently united by means of the isthmus. This statement seems to be confirmed by the condition of the gland in several of the lower animals, where the lateral lobes continue separate, lying on each side of the trachea ; and is also supported by the occasional occurrence of a similar dispo sition in the human subject.
In my own researches it has not occurred to me to observe this mode of development, perhaps because I have not examined spe cimens at a sufficiently early period ; however, in an embryonic sheep only two inches long, where the thyroid was distinctly visible, it presented the usual appearance—the lateral lobes being connected by a narrow isthmus ; the same was the case in a human foetus of 44 months ; the isthmus, however, being wider, and not appearing to be of at all more recent development than the lateral lobes. In the embryo of the sheep just mentioned, the gland was of an opaque whitish aspect, differing materially from its natural reddish colour ; it consisted principally of nuclei, with a small quantity of granular matter. Scarce any trace of a vesicular arrangement existed ; but the whole mass was surrounded by an investing membrane very nearly homo geneous in texture. In another embryonic sheep, three inches long, the thyroid was much more of its natural reddish, semi-trans lucent colour ; still there was scarcely any vesicular arrangement, the mass consisting almost entirely of nuclei aggregated together. The thyroid of the human fetus just men tioned was of the same grayish aspect as that of the smallest of the embryonic sheep; It also consisted chiefly of nuclei, but these were to some extent collected together so as to form solid globular masses (fig. 741.), not yet, however, quite definitely isolated, nor surrounded by homogeneous envelopes. This, however, seems to be the way in which the vesicular cavities are developed ; the limitary envelopes being formed around the primitive nuclei, which assume the arrangement of epi thelium. The thyroid is of larger relative magnitude during intrauterine existence and infancy than in after life, — a fact which seems rather opposed to the view which regards this gland as alternating its action with that of the brain.