ANIMALIA VERTEBRATA— VEETEBRATED ANIMALS. 



21 



described with the same radius, shall exactly fit and correspond at their extremities. 

 In each, the mind is led from one link to another by insensible shades, almost with- 

 out perceiving any interval. 



On considering each organ separately, and following it through all the species of 

 one class, we observe that its progression, within certain limits, is preserved with a 

 singular regularity. The organ, or some vestige of it, is to be found even in species 

 where it is no longer of any apparent use, except to prove that Nature strictly adheres to 

 the law of doing nothing by sudden transitions. Yet, the organs do not all follow the 

 same order of gradation. One part is found absolutely perfect in a certain animal, 

 while another part is in its most simple form. Again, on examining a different animal, 

 the relative complication of the two organs is absolutely reversed. If, therefore, we 

 were to class different species according to each organ taken separately, we should be 

 under the necessity of forming as many series as we should have regulating organs. 

 Thus, to make a general scale of complication, it would be essential to calculate 

 the precise effect resulting from each combination, which is far from being practi- 

 cable. 



As long as the gi'eat central springs remain the same, and while we confine our- 

 selves to the same combinations of the principal organs, these gentle shades of an 

 insensible gradation are found to prevail. All the animals of each of the primary 

 divisions seem formed on a common plan, which serves as the basis of all their minute 

 external modifications. But the moment that we direct our attention from one prin- 



cipal group to another, wherein different leading combinations take place, the scene 

 directly changes. There is no longer any resemblance, and an interval, or marked 

 transition, is obvious to every one. Thus, it is impossible to find in the whole Animal 

 Kingdom any two beings which sufficiently resemble each other to serve as a link 

 between the Vertebrated and Invertebrated animals. 



The Creator never outsteps the bounds which he has prescribed to himself in the 

 laws of the conditions of existence. Ever adhering to the small number of combina- 

 tions that are possible, Nature seems to deliglit in vai-ying the arrangement and struc- 

 ture of the accessory parts. There appears in them no necessity for a particular form 

 or arrangement, while it frequently happens, that particular forms and dispositions are 

 created without any apparent views of utility. It seems only suSBcientfor their exist- 

 ence that they should bo possible, that is to say, that they do not disturb or destroy 

 the harmony of the whole. These varieties augment in number, in proportion as we 

 turn our attention from the leading and essential organs to those which are less 

 important; and when we finally arrive at the external surface of the body, where the 

 laws of external Nature require that the least essential organs, and those least liable 

 to injury, should be placed, we find the number of varieties absolutely infinite. The 

 labours of naturalists have not yet succeeded in tracing all their differences, and newly- 

 discovered species are continually rising, as it were, into existence. Yet not even is 

 a bone varied in its surfaces, in its curvatures, or in its eminences, without subjecting 

 the other bones to corresponding variations. 



THE FIRST GREAT DIVISION OF THE ANIMAL KINGDOM. 



ANIMALIA VERTEBRATA— VERTEBRATED ANIMALS. 



HAVING A BONY SKELETON, CONSISTING OF A CRANIUM, SPIXAL COLUMN, AND GENERALLY ALSO OF Ll-MUS; THE MUSCLES ATTACHED TO THE SKELETON, 

 DISTINCT ORGANS OF SIGHT, HEARING, SMELL, AND TASTE, IN THE CAVITIES OF THE FACE; NEVER MORE THAN FOUR LIMBS; SEXES SEPARATE; BLOOD 

 ALWAYS RED. 



The bodies and limbs of the Vertebrated Animals are sustained by a solid 

 iVamework or skeleton, composed of separate pieces joined together, 

 and moveable upon each other. This enables them to execute their 

 movements with vigour and precision ; while tlie solid support afforded 

 i'Y the bones permits them to attain a considerable size. It is, conse- 

 qucQtly, in tliis division that the largest animals are found. 



There are three important purposes answered by the bones of Vortebratcd Animals: 

 a solid framework is supplied to the softer parts; bases are furnished upon which the 

 muscles ai'c fixed; and a pi-otection from external injury is afforded to the vital parts, 

 and to the central masses of the nervous system. 



A limit to the size of animals is fixed by the force of gravity at the earth s surface. 

 Were animals to exceed this limit, they would fall to pieces by the weight of their 

 limbs; and a certain degree of cohesion, constituting rigidity, is inconsistent with ani- 

 mal motion. This forms a complete refutation of the idle talcs of crakens, giants, 

 and other monsters. 



The nervous system of the Vertcbrata is more concentrated, and its 

 central masses are of greater size than in other animals. Their sensa- 

 tions also are more vivid and prolonged, and they possess superior intel- 

 ligence and capacity for improvement. 



The bodies of the Vertebrata are always composed of a head, a trunk, 

 and [generally] also of limbs. 



The head is formed of the cranium or skull, and of the face. The 

 former contains the brain, and in the latter are placed the receptacles 

 for the organs of sense. 



The human skull is composed of eight bones, one frontal, two parietal, one occipi- 

 tal, two temporal, one sphenoid, and onea^thmoid. The frontal bone (or os froniis)j 

 forms the entire covering of the forehead, with the upper orbit of the socket for the eye, 

 and extends towards the temples. In the infant, this bone was originally two, which 

 have gradually coalesced into one. Two parietal bones (ossa parietalia) form the 

 Bides and upper part of the skull. The occiput, or hinder bone of the head (os occi' 

 pitis), forms the base of the skull. There are two temporal bones (o.vic tempo, 

 jalia^t one on each side; and one sphenoid bone (^splienoidcs.) The principal part 

 of the sphenoid bone, and the Vihole of the ethmoid (at/imoidcs}, are placed in the 

 internal part of the skull; they support the brain, and allow the nerves to pass through 

 their irregular grooves and cavities. 



In^the skull of the fcctu?, during the earher months of its existence, cartilage is 

 Bubstituted for bone. By degrees portions of the cartilage are absorbed, and earthy 

 particles are as gradually deposited in their places from certain points, which are 

 thence called centres of ossiacation. 



The form, which was rudely defined at first, advances by gradual steps toward 

 perfection, and by definite and fixed laws. A certain system of ossifying centres be- 

 longs to each separate bone. The fibres proceeding from centres of the same bone, 

 have a natural affinity for each other, and generally refuse to unite with the fibres of 

 other bones. This, however, sometimes takes place, as in the Os frontisy when the 

 two bones are said to form an anchylosis. 



As these bones of the skull are destined to protect the brain, they are formed in a 

 manner peculiarly adapted for that purpose. Each bone is composed of two plates; 

 the internal layer (or tabula viirea) is dense, hard, and well adapted to resist a vio- 

 lent blow; the external layer is fibrous and tough, and fitted to check the vibration 

 6 



of the internal part. It has been rightly compared to a soldier's helmet, lined with 

 leather and ornamented with hair. 



The edges of the bones, where they meet together, arc beautifully dove-tailed, 

 forming sutures, which give the whole skull unity and strength. These irregular 

 lines uf junction are formed by the fibres of opposite centres, which continue to secrete 

 the bony particles, and to insinuate themselves until finally stopped by their mutually 

 increased resistance. 



Fourteen bones compose the human face. These are usually grouped together 

 under the names of upper and lower jaws. They consist of two superior maxillary, 

 two malar, two nasal, two palate, two lachr}Tnal, two turbinated, one vomer, and one 

 inferior maxillary bono. Of these, the most remarkable is the lachrymal, which con- 

 sists of a delicate bony scale, as thin as paper, and containing the passage which 

 conveys the tears from the eye into the nasal cavity. 



The relative size and arrangement of the bones of the face and skull vary consider- 

 ably in diff'erent tribes of animals. 



The head is always placed at the anterior extremity of the vertebral column, and 

 opposite to the tail. It is divided into three parts, which may vary in their relative 

 proportions, but are never wanting. These divisions are, first, the Cranium, contain- 

 ing the brain, in the partitions of which are situate the cavities of the internal ear, 

 and frequently a part of those of the noae ; secondly, the Face, terminated below by 

 the upper jaw, and containing the orbits of the eyes, with the nasal canties; and 

 thirdly, the lower jaw. 



Tlie trunk is sustained by the spine of the back and by the ribs. 



The spine is composed of vertebrEe. Each vertebra is, in general, 

 moveable on the adjoining ones. The first carries the head; and through 

 each vertebra there is an annular or ring-like part, all of which collectively 

 form a canal. In this canal is placed that portion of tlie medullary sub- 

 stance called the spinal marrow, from which the nerves proceed. 



In the spinal column of adult animals, great strength is combined with a consider- 

 able degree of general flexure. The arrangement of the solid matter of which it is 

 composed, is admirably calculated to give lightness to the whole fabric. It is neither 

 collected into one solid mass, nor generally diffused as in sponge. But the column is 

 hollow; and, by the laws of I\Iechanics, it has been clearly ascertained that wliile the 

 height and the quantity of matter of two pillars remain the same, that column is the 

 stronger which is, to a certain degree, of a hollow construction. 



The cartilages between the several vertebree yielil considerably to the pressure of 

 the body, after remaining for a long time in the vertical position, and expand after 

 repose. Hence arises the very striking phenomenon, that a man is considerably taller 

 in the morning after a night's rest, than in the evening after the fatigues of the day. 

 The long absence of pressure upon the cartilages of the vertebrae causes them to 

 expand. This difl'erence of height has been observed only in the human species; they 

 arc the only creatures who walk erect, and throw the pressure of the upper parts of 

 the body upon the back bone. Thus we find no difference in the height of horses, 

 even after the longest day's journey. 



Among young people this difi^erence is more observable than in those who ai'c more 

 aged. Persons of very laborious habits sink rather less than those of sedentary habits ; 

 and when the height is once lost, it cannot be restored for that day, not even by tho 

 use of the cold bath; and it can be alone regained by a night's repose in a horizontal 

 position.* 



* Philosophical Transactions, No. 383, p. 87. 



