1108 



THYROID GLAND. 



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 foetus 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 



Fig. 741. 



Incipient vesicle from thyroid of human foetus, at 

 about mid-period. It is a solid mass of nuclei, not 

 enclosed in a distinct envelope. 



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 intra-uterine 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. 



COMPARATIVE ANATOMY OF THYROID. The 

 existence of a thyroid gland in all the Mam- 

 malian orders seems to be undoubted ; and 

 though it is probable that, by a close and ex- 

 tended survey of the various families, some 

 interesting and perhaps instructive peculiarities 

 might be detected, yet I have not the oppor- 

 tunities necessary for undertaking such an 

 inquiry, and can only record one observation 

 where some deviation from the ordinary con- 

 dition was discovered. This was in a Rabbit, 

 in which the organ presented the vesicular 

 arrangement much less manifestly than is 

 usually the case. The epithelium consisted of 

 small imperfect celloid particles, disposed so 

 as to form hollow spheres ; but there was 

 scarce any appearance of secretion in the 

 included cavities, which were small, and 

 might at first have escaped observation. 



For the following summary of the compa- 

 rative anatomy of the thyroid in birds and rep- 

 tiles, I am principally indebted to Mr. Simon's 

 paper in the Philosophical Transactions. 



In Birds, there are found, in all the va- 

 rious orders, two glands, situated one on 

 each side of the trachea, very near the lower 

 larynx, and frequently attached to the jugular 

 veins. They possess the characteristic struc- 

 ture of the thyroid body, consisting of a 

 dense aggregation of closed vesicles, which 

 contain a kind of epithelium, and are invested 

 by a close capillary network applied over their 



homogeneous envelope. The position of these 

 glands with respect to the larynx seems to be 

 neither essential nor constant; it is however 

 stated by Mr. Simon, that it " always corre- 

 sponds to a particular spot of the vascular 

 system, viz., that it lies on the cervical vessels, 

 and receives its supply of blood just opposite 

 to the point at which the vertebral or carotid 

 arteries diverge to their respective destina- 

 tions." 



The following are the details of a mi- 

 croscopic examination which I made of these 

 glands in a Pigeon. They consist of closed 

 vesicles about -^^ inch diameter, having 

 their homogeneous envelope lined by a rather 

 thick layer of epithelium, so that the cavity 

 is proportionably contracted, sometimes not 

 being more than ^-^_ inch diameter The 

 epithelium consists, for the most part, of nuclei 

 and granular matter. Some of the nuclei are 

 very perfect, and show a distinct vesicular struc- 

 ture, with a well marked peripheral nucleolus. 

 Others are more like granules, solid and opaque, 

 and not above half the size. The nuclei are 

 imbedded in granular matter, which for the 

 most part is diffused freely about, but occa- 

 sionally, though very rarely, constitutes the 

 contents of a cell. In some cavities the epi- 

 thelium assumes the form of non-nucleated 

 delicate vesicles, of rather large size (fig. 1^2. b.). 

 No secretion, capable of being recognised 

 by the eye, exists in the glandular cavities. 



Fig. 742. 



From the thyroid of a Pigeon. 



a, 'b, Two vesicles. The epithelium of a consists 

 of nuclei and granular matter ; that of b of rather 

 large, delicate, pellucid vesicles. The central cavity 

 is small, especially in a. 



c, The two varieties of nuclei, and 



d, A complete nucleated cell. 



Reptiles. In the order Chelonia the organ, 

 which is demonstrated by the microscope 

 to be really the thyroid, is found occu- 

 pying a definite and uniform position. It lies 

 in the median plane of the body, immediately 

 above the base of the heart, between the two 

 carotid arteries, a ndis overlapped and con- 

 cealed by the pericardial lobes of the thymus. 

 The structure of the gland in a young Tor- 

 toise I found extremely well marked; the ve- 



