Royal Society, 145 



formed by the adherent surfaces of contiguous cells. The diameter 

 of the fibril, in the state of relaxation, is the 20,000th part of an 

 inch. The cells are filled with a transparent substance, to which 

 the author gives the name of Myoline, and which differs in its re- 

 fractive density in different cells. In four consecutive cells the 

 myoline is of greater density than in the four succeeding cells, and 

 this alternation is repeated throughout the whole course of the fibril. 

 In consequence of all the fibrils composing the ultimate fasciculus 

 having the same structure, and the cells, which are in lateral juxta- 

 position, containing myoline of the same density, they act similarly 

 on light, and the whole presents, to the eye of the microscopic ob- 

 server, a succession of striae or bands, dark and luminous alternately, 

 and transverse to the direction of the fasciculus; an appearance 

 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 Avill 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, exocelus, 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- 



Phil. Mag. S. 3. Vol. 25. No. 164.. August 1844. L 



