22 MAMMAUA. 



NERVOUS Si'STEM. 



THE coverings of the brain and spinal-cord agree in general, in so 

 far as the arachnoid and pia mater with its net-work of vessels are 

 concerned, with those of Man. The dura mater usually forms a falx, 

 which extends deeply between the hemispheres. As a general 

 rule, the tentorium cerebelli is present, but the falx minor is nearly 

 always wanting, on account of the large vermiform lobe of the cere- 

 bellum projecting beyond its hemispheres. The tentorium is in many 

 Mammalia supported by a bony plate springing from the internal 

 surface of the skull, and is particularly strong in the Cat, and other 

 Ferae, but feeble in the Horse, Dolphin, and some of the Apes. An 

 osseous plate is seldom found in the falx, as in many Birds ; but it 

 occurs in the Ornithorynchus. Between the laminae of the dura 

 mater are found the venous sinuses. 



The spinal cord of the Mammalia extends considerably lower 

 than in Man ; as a rule, it reaches as far as the sacrum, though 

 in the Cetacea it appears to terminate higher up. The nerves 

 themselves of the Cauda equina pass out through the openings 

 of the most complete of the caudal vertebrae. Of the two enlarge- 

 ments upon the cord, the posterior is wanting where there is imper- 

 fect development of the hinder extremities, as in the Cetacea ; 

 and sometimes the two enlargements coalesce so as to form a single 

 one of very considerable size. The central canal which is present 

 in the foetus of Man (perhaps also in the adult) appears in very 

 many of the Mammalia to exist during their whole life ; at all 

 events, the fourth ventricle is more or less deeply prolonged into the 

 spinal marrow. The brain is developed in the lowest degree, and 

 is truly bird-like in the Ornithorynchus, where the pons is very 

 small, and there is only a rudiment of the corpus callosum present, 

 as in the Marsupiata, while the hemispheres of the cerebellum 

 appear more as appendages, or lateral extensions, of the very greatly 

 developed vermiform lobe ; the corpora quadrigemina form only a 

 pair of ganglia, the posterior pair being scarcely visible ; the optic 

 thalami coalesce in the middle by a very strong commissura mollis, 

 and the hemispheres are without convolutions. In the Rodentia, 

 Marsupiata, and Edentata, the vermiform portion of the cerebel- 

 lum is so considerable, that the hemispheres appear to recede 

 very much ; they are already more developed in the Ruminantia 

 and Pachydermata, and are still more highly organized in the Car- 



