128 MAMMALIA. 



and therefore does not require any other opposing power. It gives origin to the third 

 nerve, the smaller portion of the fifth, the hard portion of the seventh, and the ninth. 

 The median or inner layer has been apportioned to the flexors, the first section to 

 the inner side of the restiform body for inspiratory motions, the second for the flexors 

 of the upper extremity ; the third forming the median crescentic tract or belt, for 

 the flexors of the lower extremity and spine. As the tendency of the joints is to 

 flexion, and as the flexor muscles have also to be countervailed by an opposing 

 power, a much larger force is required for the extensor muscles. The bulk of the 

 outer layer of convolutions is therefore greater, nearly twice as much as that of those 

 of the inner layer for the flexors. The first section of the outer layer has been 

 appropriated to the extensors of the lower extremity ; the second to the extensors of 

 the upper extremity ; the third to the outer portion of the restiform body for expiratory 

 processes, and the outer crescentic belt to the extensors of the spine. 



CEREBRAL NERVES. The origin of the olfactory nerve varies in different 

 kinds of animals ; in simise it is very similar to that in man, but in the horse, sheep, 

 dog, and others, it occupies a very considerable space at the base of the brain, and is 

 connected with the inferior portion of the hippocampus. It terminates in a bulb 

 composed principally of cineritious matter placed over the cribriform plate of the 

 ethmoid bone ; and through perforations of this, numerous branches pass to the nose, 

 where they become more or less combined with branches of the fifth ; the branches 

 of one portion, after an expansion on the membrane covering the plates of the ethmoid 

 bone, converge and become connected with a branch of the lateral nasal nerve, and 

 then separate to be distributed on the membrane covering the turbinated bones and 

 the rest of the exterior walls of the nose ; the other portion is continued upon the 

 membrane expanded on the septum ; the branches of the nerves, soon after their 

 entrance into the nose, become impacted in the membrane, copiously supplied by 

 arteries, and having underneath numerous large veins so as to give it a cavernous 

 appearance by which it can be made more or less tense by modifying the act of 

 respiration. This mode of distribution is well seen in the horse. 



The optic nerve in mammalia is very similar to that in man. The tract is a 



