515 



which has been noticed by preceding observers, but of which the 

 cause had not hitherto been ascertained. A dark stria may occa- 

 sionally appear as a luminous one, and vice versa, when viewed by 

 light transmitted at different degrees of obliquity. 



The structure here described, the author remarks, reduces the 

 muscular fibre to the simple type of organization exhibited in the 

 combination of a series of cells, associating it with other tissues of 

 cell formation, and will probably, he thinks, open new sources of 

 explanation of the immediate agency of muscular action, a power 

 hitherto involved in the deepest mystery. 



2. "On the Comparative Anatomy of the* Thyroid Gland." By 

 John Simon, Esq., Assistant Surgeon to King's College Hospital, 

 and Demonstrator of Anatomy in King's College. Communicated 

 by Joseph Henry Green, Esq., F.R.S. 



The author, considering that the careful dissections of Meckel 

 and Cuvier have fully established the universal existence of a thyroid 

 gland in the whole of the class Mammalia, proceeds to consider the 

 comparative anatomy of this organ in the remaining classes of ver- 

 tebrated animals. His dissections of birds have included all the 

 orders, and, in most instances, several families from each : he has 

 never failed to find in them a thyroid gland, and, with the aid of the 

 microscope, to recognise its peculiar structure ; he presumes, there- 

 fore, that it is universally present in that class of animals. He has 

 also detected the presence of this organ in reptiles of every order ; 

 although generally either wholly overlooked by anatomists, or mis- 

 taken for the thymus. Descriptions are here given of its appearance, 

 position and structure in different families of Chelonia, Sauria, 

 Ophidia and Batrachia. In the class of Fishes, it is by no means 

 universally or even generally present. The author has found it in 

 the carp, anableps, pike, exocetus, cod, haddock, whiting, eel, stur- 

 geon, callorhynchus, shark and skate, and perhaps in the lamprey. 

 On the other hand, it appears to be absent in the perch, mullet, 

 gurnard, mackerel, tench, salmon, trout, herring, plaice, halibut, 

 turbot, sole, cyclopterus, gymnotus and balistes. 



The general conclusion which the author deduces from his re- 

 searches is, that the distribution of the thyroid gland is regulated by 

 a simple and uniform law ; being dependent on the existence or non- 

 existence of another organ with which its presence alternates, and 

 which, in many fishes, assumes the form of a minute supplementary 

 gill, the vessels of which communicate, on the one hand, with the 

 systemic veins about the base of the cranium, and on the other, by 

 a single long trunk with the first branchial vein. 



Although the thyroid gland occupies various situations in differ- 

 ent animals, it always maintains an intimate relation with the vas- 

 cular supply of the brain, and is always so nourished as to be 

 capable of a greater or less nutrition according to the activity or 

 repose of that nervous centre. 



3. " On the Resolution of Numerical Efpiations." By Joseph Agar, 



