67 



Mr. John Green exhibited under the microscope a por- 

 tion of epidermis from the border of the mouth of Pristis 

 sag-ittata, containing teeth in immediate juxtaposition 

 with the placoid scales which cover the body of the fish. 

 He called attention to the perfect identity of structure in 

 the two cases, viz : a cavity or depression in the under 

 surface of the tooth or scale, and tubes ramifying in its 

 substance. The only difference is in size and form, the 

 teeth being two or three times the size of the scales, and 

 triangular in outline, while the scales are nearly circular. 



The President announced the presence of Dr. Brown- 

 Sequard, and invited him to address the Society. 



Dr. Brown-Sequard expressed his thanks for the cour- 

 tesy extended to him, and proceeded as follows : — 



Experiments have lately been made to determine if the intro- 

 duction of air into the chest, through the respiratory passages, 

 with great force, does not have some inttuence upon the action of 

 the heart ; and it has been found that there is a diminution in the 

 frequency of the heart's action, sometimes to such an extent, that 

 the pulsations amount to only two thirds the normal number. 



The exi^lanation which has been given to tliis phenomenon is 

 based upon mechanical grounds, but M. Sequard thinks if this 

 explanation be correct, it is so only to a certain extent. One 

 other cause at least, that of a nervous influence, he has demon- 

 strated to exist. 



It is well known that some mammals resist asphyxia for a long 

 time, making efforts to breathe, often for several hours, when the 

 entrance to the lungs is closed. Now it is found that when the 

 chest of an animal in this condition is opened and retained in 

 such a position that there can be no motion and no mechanical 

 action, the diminution of the pulsations persists. The phenome- 

 non is then due to the action of the par vagum nerve. It is 

 found that when this nerve is irritated at its root or galvanized, 

 the action of the heart is arrested ; in this respect differing from 

 the other muscular organs, which are excited to action by irrita- 

 tion or galvanization of their nerves. When the chest is opened. 



