233 Sir E. Home's Additions to an Account 



mater nearly resembles that of the human brain ; it becomes 

 thicker where it covers the spinal marrow. The space between 

 the dura and pia mater is occupied by a cellular membrane of 

 a very fine texture. 



As the different parts of the brain are described in the ex- 

 planation of the drawing, I shall only remark in this place, 

 that the circumstance most deserving of observation respecting 

 it is, that the cerebellum has an increase of size in the Squalus 

 Maximus in a much greater degree beyond that of the Squalus 

 Acanthias than the tubercula quadrigemina. The protuberances 

 from which the olfactory nerves arise were probably large in 

 the same proportion with the tubercula ; at least, in the brain 

 of a shark, preserved in the Hunterian Collection, of a smaller 

 size than that of the Squalus Maximus, but much larger than 

 that of the Squalus Acanthias, that is the case. 



The eye is small for the size of the fish ; the ball has pro- 

 jections on the sclerotic coat, where the muscles are attached, 

 which make it approach to a quadrangular form ; but its in- 

 ternal cavity is circular. The circumference in the widest 

 part is nine inches. The longest diameter three inches, the 

 shortest one inch and three quarters. The sclerotic coat is 

 cartilaginous, one quarter of an inch thick on the posterior 

 part, becoming thinner towards the ciliary processes, where 

 it is only one- sixteenth of an inch. 



The cornea is thin, but made up distinctly of three layers, 

 of which the middle one is by much the thinnest. The optic 

 nerve is nearly of the size of the sixth pair, and, where it per- 

 forates the sclerotic coat, projects a little before it gives off' the 

 retina, which is extremely thin. 



The choroid coat is covered with a tapetum lucidum of the 



