THYROID GLAND. 



1107 



covered only by the deep fascia and platysma ; 

 but it soon turns vertically downwards, and 

 runs beneath the sub-hyoidean muscles to the 

 upper extremity of the gland, where it divides 

 into three branches; one of these runs be- 

 tween the thyroid gland and the trachea, a 

 second skirts the external border of the lateral 

 lobe, while the third, running along the inter- 

 nal border, forms an anastomosis with the cor- 

 responding branch of the opposite side. 



The inferior thyroid is noticed by Cruveil- 

 hicr as one of the arterial branches most 

 liable to vary in its origin, an opinion which, 

 emanating from a less high authority, I should 

 have been inclined to question. It may arise, 

 acccording to him, from the common carotid, 

 the arch of the aorta, or the arleria innomin- 

 ata. (The supra-scapular often, less commonly 

 the posterior scapular, and sometimes even 

 the internal mammary, spring from the com- 

 mencement of the inferior thyroid, which is 

 therefore called the thyroid axis.) Its course 

 is peculiar ; it runs at first straight upwards, 

 then comes downward, and again ascends to 

 reach the inferior extremity of the lateral lobe 

 of the gland. It passes in front of the trachea, 

 and behind the great vessels and vagus nerve : 

 the connecting cord of the sympathetic descends 

 behind it to the middle cervical ganglion when 

 it exists, which is then almost constantly 

 found, as it were, seated astride upon the 

 vessel, exactly on the convexity of its first 

 curve. Like the superior thyroid, it has three 

 terminal branches, one running along the in- 

 ferior border of the gland, another breaking up 

 over the posterior face of the lateral lobe, and 

 a third which penetrates between the gland 

 and the trachea, and anastomoses with the 

 one of the opposite side along the upper 

 border of the isthmus. (Cruveilhier). 



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 g-.^o mcn > a "d the interspaces 

 were, I think, two or three times as large. 



There are corresponding vence cornites 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 into the vena tramversa, 

 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 arter) r . 



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 caelio-spinal tubules, 

 running for 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 

 4 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 

 4s 2 



