577 



exceedingly varied and beautiful, and especially characteristic of the 

 species in which they occur. 



The author has named and figured the whole of the spicula de- 

 scribed in the paper, and has traced some of the most complicated 

 ones from their earliest and simplest state, through all the stages of 

 their development to the adult condition. More than a hundred 

 distinct forms of these organs are thus described, so as to render 

 them available hereafter to naturalists as characteristic of species. 



XXVI. " Researches on the Intimate Structure of the Brain ; 

 Human and Comparative. Part I. The Medulla oblon- 

 gata." By J. LOCKHART CLARKE, Esq., F.R.S. Received 



June 18, 1857. 



(Abstract.) 



The medulla oblongata, as described in this memoir, extends from 

 the first cervical nerve to the lower border of the pons Varolii. Of 

 its elementary parts, the author first traces the arciform fibres, which 

 may be divided into a superficial and a deep layer. Those of the 

 superficial layer may in turn be divided into three sets. The 

 connexions of these are first followed out in detail ; the fibres of the 

 deep layer are described further on. In all mammalia these fibres 

 are very distinct, but less intricate than in man. They may be 

 found also in birds, reptiles, and fishes. 



The anterior pyramids are found to be composed of four orders of 

 fibres : 



1. Decussating fibres from the lateral columns, forming their chief 

 bulk. 



2. Decussating fibres from the posterior columns and posterior 

 grey substance, chiefly at the upper part. 



3. Decussating fibres from the anterior grey substance. 



4. 2Vbn-decussating fibres of the anterior columns, separate on 

 their outer side, and on their inner side incorporated with those which 

 form the decussation. 



In mammalia generally the decussating fibres are much less 

 numerous than in man. In birds there is an evident but feeble 

 decussation. 



