24 



THE FIRST GREAT DIVISION. 



to check itself in tllis imaginary career, it actually effects a movement in the opposite 

 direction. Dr Mayo describes with accuracy the phenomena attending injuries to 

 tlie Cerebellum in various animals. " The removal of the Cerebellum in fish produces 

 no further immediate effect than that of weakening the animal ; and frogs, from which 

 this organ is removed, show an indisposition to move, unless irritated or placed in 

 water, when the movements, though less lively than before, are not observed to be 

 o',hervvise affected. In Birds and in Mammalia, more important results ensue upon the 

 injury or removal of the Cerebellum, which, it may be remarked, does not appear to 

 be sensible to pain from mechanical lesion. If the Cerebellum be wounded upon one 

 side, the animal appears to be generally weakened upon the same side : if the wound 

 be deep, the body upon the injured side is rendered paralytic. If, in a rabbit, the 

 upper and middle portion of the Cerebellum be removed, the hind legs are observed 

 to he spread, the fore legs are extended forwards in a state of rigidity ; the whole 

 attitude is that of preparation for moving backward, or throwing itself over. After 

 a short time, the animal beats the ground with its fore paws, the hind legs not moving, 

 and urges itself backwai-ds. The flight and wallc of pigeons are not affected by the 

 removal of the upper part of the Cerebellum. After a deep section has been made, 

 the bird totters, falls on its breast, rises again, and is in continual agitation. A deeper 

 section still causes it to walk and fly backwards. After the entire removal of the 

 Cerebellum, the bird, when irritated, walks as usual : when thrown into the air, it 

 moves its wings regularly, and lights upon its feet. M. Magendie mentions the case of 

 a young woman, who was affected wdth a nervous malady, that forced her to run rapidly 

 backwards, disregarding every peril. If, in a rabbit, a section through the middle 

 portion of the Cerebellum be made, in the median plane, the eyes of the animal are 

 observed to be in extraordinary agitation, and as if stai'ting from their sockets : the 

 animal inclines towards one side, then is suddenly thrown towards the opposite, as if 

 unable to balance itself with precision : its fore legs are rigidly extended forwards, as if 

 it were in the act of receding. If a vertical section of the Cerebellum be made, leaving 

 one-fourth of the whole adhering to the cms (or shank) of the right side, and three- 

 fourths to the left, the animal rolls over and over incessantly, turning itself towards 

 the injured side. The right eye is directed downwards and forwards, the left eye 

 upwards and backwards. On malting a similar section upon the left side the animal 

 stops, and the eyes resume their natural direction." 



3. The Ceueeruji, or larger lobes of the brain, arise from the expanded portions 

 of the anterior or fore columns of the spinal mai'row'. When certain portions of the 

 Cerebrum are divided in various animals, they spring suddenly forwards, and continue 

 to advance steadily in a straight line. Even when opposed by some obstacle, they 

 continue to preserve the attitudes of one advancing. 



4. The Optic Tubercles give rise to the optic nerves, and are formed by a pro- 

 duction of the central columns of the spinal marrow. Upon injm'ing this pai-t of the 

 brain, blindness immediately follows. 



The relative arrangement of these several parts of the brain differs considerably 

 among the several classes of animals. In Fish, they are arranged nearly in a straight 

 line, while in the Mammalia and other higher tribes, they are disposed in a more com- 

 plicated manner. The relative proportions also vary. In the higher animals, the 

 cerebral hemispheres ai-e much larger, in proportion to the tubercles, than in fishes. 



The Vertebrated animals have always two jaws. The lower jaw 

 possesses the greatest power of motion, and may be either raised or de- 

 pressed. The upper one is, in general, entirely fixed. 



The Upper jaw is immoveable in Man, in Quadrupeds, and in some Reptiles, 

 as the Tortoise and Crocodile ; but it is more or less moveable in Birds, Serpents, 

 and Fishes. The lower jaw is always moveable in the Crocodde, although the con- 

 trary has been asserted. 



Both jaws are almost always armed with teeth. These are excrescences 

 of a peculiar nature, nearly resembling bone in their chemical composi- 

 tion, but which grow by the deposition of matter in certain sacs. Yet 

 one entire class of Vertebrated animals (Birds), have their jaws covered 

 with a horny substance ; and among the Reptiles, one entire genus (Tor- 

 toises) are similarly supplied. 



The teeth are used by the various classes of Vertebrated animals for different par- 

 poses. In general they are for masticating the food ; often for weapons of defence ; 

 in some they are employed for digging and for seeking out the food; and in others, 

 they seem designed for no other purpose than for defending the eyes, as in the Phaco- 

 chcerus iEthiopicus, or African boar. 



The formation of teeth proceeds upon a plan entirely difl"erent from that employed 

 by Nature in the deposition of bone. It will be recollected that the bones are per- 

 vaded in every direction by vessels which nourish, renovate, and absorb their particles ; 

 but the teeth, on the contraiy, are almost entirely destitute of vessels. When once 

 deposited, they remain in a certain degree unchanged; and hence, when once de- 

 stroyed, they cannot be renewed. The foundations of the teeth are laid before birth, 

 and each tooth is formed in a small sac, by the deposition of earthy matter. They 

 are covered by the e7tamel, which is the hardest animal substance in nature, and 

 will even strike fire from steel. 



The structure of the teeth in graminivorous animals is peculiar. A grinder is 

 composed of several distinct teeth, each tooth being covered with its own enamel, 

 and tlie whole united together by a kind of cement. 



The int stinal canal of the Vertebrated animals proceeds from one 

 extremity of the body to the other, undergoing various bendings, with 

 several expansions and contractions. It possesses subsidiary organs, and 

 receives various secretions, having a dissohing power. Some of these 

 are seated in the liiouth, and called the saliva; others, formed only 

 in the intestines, bear several names. The two most important secre- 

 tions are the Juice formed by the gland called the Pancreas, and the Bile, 

 which is produced from another very large gland called the Liver. 



After the food has been digested, it passes into the alimentary canal. 

 That portion fitted for nutrition, called the chyle, is absorbed by parti- 

 cular vessels called Lacieals, and transported into the veins. After the 

 several parts of the body have been nourished by the blood, the nutri- 

 tious particles remaining unabsorbed are also introduced into the veins 

 by vessels analogous to the Lacteals, and forming with them an arrange- 

 ment called the Lijmphaiic system. 



The veins bring back to the heart the blood that has served to nourish 

 all parts of the body, and which has just been supplied with chyle and 

 lymph. But before it is in a proper state again to be transported by the 

 arteries throughout the body, it is obliged to pass wholly, or in part, 

 through the organ of respiration. In the three highest classes [Mam- 

 mjJia, Birds, and Reptiles] the respiration is pulmonary, or performed 

 through Lungs, consisting of an assemblage of small cells through which 

 the air penetrates. In the Fishes alone, and in certain Reptiles during 

 the first periods of their existence, the respiratory organ consists of 

 GiUs, composed of a series of thin plates between which the water flows. 



In all the Vertebrated Animals, the blood supplying the liver with ma- 

 terials for the bile, is derived from that venous blood which has partly 

 circulated in the coats of the intestines, and partly in a particular organ 

 named the Spleen. After being collected in a canal termed the Vena- 

 portiB, this blood is again subdivided at the liver. 



Toils ces animaux ont aussi une secretion particulierc, qui est celle de 

 I'urine, et qui se fait dans deux grosses glandes attachees au.x cotes de 

 I'epine du dos, et appelees 7-cins: la liqueur que ces glandes produisent, 

 sejourne le plus souvent dans un reservoir appele la vessie. Les sexes 

 sont separes ; la femelle a toujours un ou deux ovaires, d'ou les oeufs se 

 detachent au moment de la conception. Lc m?de les feconde par la li- 

 queur seminale; mais le mode de cette fecondation varie beaucoup. 

 Dans la plupart des genres des trois premieres classes, elle exige une 

 intromission de la liqueur ; dans quelques reptiles, et dans la plupart des 

 poissons, elle se fait quand les CEufs sontdeja pondus. 



DIVISION OF THE VERTEEE.iTED ANIMALS INTO EOtJIt CLASSES. 



1. Mammalia (Man and Beastz) 2. Aves (^Birds) 3. Repiilia (ReptilesJ 



4. Pisces (Fishes). 



We have just explained the several points in which all the Vertebrated 

 Animals resemble each other. There are, however, certain differences, 

 which give rise to their separation into four large subdivisions or classes. 

 These are characterized by the particular manner in which their motions 

 are performed, or by the degree of their energy or vigour; and these 

 again depend upon the quantity of their respiration. The muscular fibres 

 possess a greater or less degree of irritability and general energy, according 

 as the respiratory organs are more or less perfect. 



There are two conditions which determine the quantity of Respiration. 

 The first is, the relative quantity of blood supplied to the respiratory 

 organ in a given time ; and the second is, the relative quantitj' of oxj'gen, 

 entering into the composition of the surrounding fluid. The quantity of 

 blood, purified by respiration, depends upon the arrangement of the organs 

 adapted for respiration and for circulation. 



The organ of circulation [or heart] may be either double or single. 

 The entire blood, arriving from all parts of the body through the veins, 

 may be obliged to circulate through the respiratory organ, before being 

 again distributed by means of the arteries ; this occurs when the heart is 

 double : or a part only of the blood returning from the body may be 

 obliged to traverse the respiratory organ, while the remainder returns 

 through the body, without having been submitted to the action of respira- 

 tion — which is the case when the heart is single. The latter arrangement 

 occurs in Reptiles ; and the quantity of their respiration, with all the 

 qualities depending thereon, is determined by the quantity of the blood 

 transmitted to the lungs at each pulsation. 



The circulation of the warm-blooded animals, possessed of a double heart [being 

 Mammalia and Birds] is performed in the following manner : — The blood is propelled 

 from the left ventricle into the aorta, and thence diflused throughout the body by 

 means of the arteries. It passes through the minute capillary vessels at the extremities 

 of the arteries into the veins ; from which it slowly collects in the vena cava, and is 

 deposited in the right auricle of the heart, and thence removed into the right ventricle. 

 It is then propelled by the right ventricle through the pulmonary artery into the lungs, 

 where it receives the action of the air ; and being restored by the pulmonaj-y veins back 

 :^gain to the left .auricle of the heai't, it passes thence into the left ventricle, prepared 

 again to resume its circuitous course. 



The force exercised by the heart upon the blood, and the velocity of the circulation. 



