MAMMALIA. 



THE BRAIN. The brain is strongly surrounded and supported by the dura 

 mater ; it is closely invested by the pia mater, by which its bloodvessels are conducted, 

 and it is more or less loosely enveloped by the arachnoid. The dura mater forms a 

 sheath for the nerves at their exit, and then becomes very thin and apparently lost on 

 the neurilema ; the pia mater is continued on the nerves, and forms the connection of 

 the fibrils and a support for the vessels ; the arachnoid only loosely surrounds some 

 nerves to their exit through the foramina. 



The brain is composed of two different substances, as in man, which have a 

 similar arrangement, but the white matter appears to be greater in proportion to the 

 grey. The number and breadth and depth of the convolutions vary in animals 

 of the same kind, and those of the two hemispheres, as in man, do not always 

 correspond ; generally they are separate, and aggregated on the surface, but some- 

 times combined, as in the hedgehog and rat, also more or less even in simiae. The 

 shape of the brain varies, being rounded or oval, or more or less narrow anteriorly 

 or posteriorly, or rising to a greater height from the base. The posterior parts of 

 the hemispheres cover the cerebellum in simiae, but in some not entirely, as in the 

 porpoise, jaguar, dog, and fox; in others still less, as in the horse and sheep; not 

 at all in the hedgehog ; whilst in the rat, a portion of the quadrigeminal bodies is 

 exposed. The superior parts of the hemispheres in some animals faintly exist, but in 

 all are generally much smaller than in man, and particularly in their proportion to the 

 oblong and spinal medulla and the nerves ; their greater or less separation depends 



