Book IL NERVOUS STRUCTURE OF ANIMALS. 287 



forwards, backwards, and laterally. In those, which are remarkable for this faculty, 

 the thighs of the hind-legs are in general of uncommon size and strength. Among 

 reptiles the leaping frog is well known, in opposition to the crawling toad. Among 

 quadrupeds, those are observed to leap best, which have the hind legs longer and thicker 

 than the fore legs, as the kangaroo and the hare. These walk with difficulty, but leap 

 with ease. 



1869. Serpents are said to leap, by folding their bodies into several undulations, which they unbend 

 all at once, according as they wish to give more or less velocity to their motion. The jumping maggot, 

 found in cheese, erects itself upon its anus, then forms its body into a circle, bringing its head to the 



tail ; and, having contracted every part as much as possible, unbends with a sudden jerk, and darts 

 forward to a surjirising distance. Many crabs and podurae bend their tail, or hairs which supply it* 

 place, under their belly, and then suddenly unbending, give to the body a considerable degree of pro- 



forward to a surjirising distance. Many crabs and podurae bend their tail, or hairs which supply it* 

 place, under th ' " " 

 gressive motion. 



1870. Flying. Flying is the continued suspension and progress of the whole body in 

 the air, by the action of the wings. In leaping, the body is equally suspended in the 

 air, but the suspension is only momentary. In flying, on the contrary, the body remains 

 in the air, and acquires a progressive motion by repeated strokes of the wings on the 

 surrounding fluid. The centre of gravity of the bodies of flying animals, is always 

 below the insertion of the wings, to prevent them falling on their backs, but near that 

 point on which the body is, during flight, as it were suspended. The action of flying is 

 performed by animals belonging to different classes. Among the Mammalia, bats dis- 

 play this faculty, by means of wings, formed of a thin membrane extending between 

 the toes, which are long and spreading, the fore and hind legs, and between the hind 

 legs and the tail. In birds, the wings, which occupy the place of the anterior extremi- 

 ties in the tnammalia, and are the organs of flight, consist of feathers, which are stronger 

 than those on the body, and of greater length. Among reptiles, the flying lizard may be 

 mentioned, whose membranaceous wings, projecting from each side of the body, without 

 being connected with the legs, enable it to fly from one tree to another in search of food. 

 A few fishes are likewise capable of sustaining themselves for a short time by means of 

 their fins ; these are termed flying fish. Spiders are able to move in the air by means 

 of their threads. 



1871. Swimming is the same kind of action in water, as flying is in air. The organs 

 which are employed for this purpose, resemble the oars of a boat in their mode of action, 

 and in general possess a considerable extent of surface and freedom of motion. Swim- 

 ming, however, is not confined to those animals which are furnished with oars or 

 swimmers. Many animals move with ease in the water by means of repeated undulations 

 of the body, as serpents, eels, and leeches ; or by varying the form of the body by 

 alternate contractile and expansive movements, as the medusae. 



1872. In these different displays of voluntary motion, the muscles are only able to 

 continue in exercise for a limited period, during which their irritability diminishes, and 

 the further exertion of their powers becomes painful. When thus fatigued, animals 

 endeavor to place themselves in a condition for resting, and fall into that state of 

 temporary lethargy, denominated sleej)' 



1873. The positions assumed hy animals during sleep, are extremely various. In the 

 horse, they even differ according to circumstances. In the field he lies down, in the 

 stable he stands. Dogs and cats form their bodies into a circle, while birds place their 

 heads under their wings. 



1874. The ordinary shape of sleep is likewise exceedingly various in different animals, and in the same 

 animal is greatly influenced by habit. It in general depends on circumstances connected with food. It is 

 probable, that all animals, however low in the scale, have their stated intervals of repose, although we are 

 as yet unac(juainted either with the position which many of them assume, or the periods during which 

 they repose. 



SuBSECT. 3. Structure of the Nervous System,. 



1875. The nervous system, by containing the organs of sensation and volition, is that 

 which distinguishes animal from vegetable beings. It consists in the vertebral animals, 

 of the brain, the spinal marrow, and the nerves. 



1876. The brain, exclusive of its integuments, appears in the form of a soft, compres- 

 sible, slightly viscous mass. The spinal marrow originates with the brain, and consists of 

 four cords united in one body. The nerves, also, originate in the brain or spinal mar- 

 row. Some of them appear to have a simple origin ; but in general, several filaments, 

 from different parts of the brain or spinal marrow, unite to form the trunk of a nerve. 

 This trunk again subdivides in various ways ; but the ramifications do not always ex-: 

 hibit a proportional decrease of size. It frequently happens, that the branches of the 

 same or of different nerves unite and separate repeatedly within a small space, forming a 

 kind of net- Work, to which tlie name plexus has been applied. Sometimes filaments pass 

 from one nerve to another ;. and, at the junction, there is usually an enlargement of 

 medullary matter termed a ganglion. Numerous filaments, from different nerves, often 



