BRAIN AND NERVES. 265 



it gives out some gelatin. When steeped in nitric acid it be- 

 comes soft, and exhibits concentric coats of thin opaque brown 

 membranes, of a ligneous aspect. It contains no earthy salt. 

 With potash it forms an animal soap, and possesses nearly the 

 properties of horn. 



The stems of the Gorgonia umbraculum and verrucosa resemble 

 that of the Gorgonia antiphates ; but these are both provided 

 with a cortex composed of membrane and carbonate of lime. 



The Antiphates ulex and myriophyla resemble almost exactly 

 the horny stem of the Gorgonia antiphates. 



Mr Hatchett analyzed many species of sponges, but found 

 them all similar in their composition. The Spongia cancellata, 

 oculata, infundibuliformis, palmata, and officinalis, may be men- 

 tioned as specimens. They consist of gelatin, which they gra- 

 dually give out to water, and a thin brittle membranous sub- 

 stance, which possesses the properties of coagulated albumen. 

 Hence the effect of acids and alkalies on them. 



The Alcyonium ficus, asbestinum, and arbor eum, resemble very 

 much the cortex of the Gorgonia suberosa in their composition. 

 They yield a little gelatin to water. In nitric acid they soften, 

 and appear membranous. The acid takes up the carbonate of 

 lime, and likewise a little phosphate, at least when the substance 

 has been previously heated to redness. 



CHAPTER VIII. 



OF BRAIN AND NERVES. 



THE brain, that wonderful part of the human body upon 

 which the exercise of the different senses and of the understand- 

 ing depends, is situated within the cranium, and is usually di- 

 vided into the cerebrum and the cerebellum, or the brain, and the 

 little' brain. The cerebrum is situated farthest up, and is the 

 part of the brain which comes into view when the parietal and 

 frontal bones are removed. In an adult individual it is about 

 eight times the size of the cerebellum. 



\ The brain is enveloped in three membranes, which have re- 

 ceived the names of the dura mater, pia mater, and arachnoid mem- 



