IN THE LIMBS OF MAMMALIA. 61 



We have seen above that the vertebrate body is composed of four regions — head, thorax, 

 abdomen, and pelvis. The tail is a pelvic prolongation, and not a distinct region, for though 

 in the lowest class it constitutes so large a part of the whole body, and is the chief agent in 

 locomotion, yet it contains no viscera, becomes more and more abbreviated, and at last dis- 

 appears as we ascend, or, according to Prof. Dana, as the vertebrates become " cephalized." 

 The number four at once furnishes a basis for a twofold longitudinal division, the head 

 and thorax in front, the abdomen and pelvis behind ; the head and pelvis are the extreme, 

 and the thorax and abdomen the intermediate, antero-posterior representatives or anti- 

 types of each other. Superficially or physiologically, the thorax seems better to repeat the 

 pelvis ; but this is due to the fact that in most vertebrates the anterior extremities are 

 shifted back upon the sides of the thorax : which, of course, cannot be of importance in a 

 morphological point of view ; so that, so far as the limbs are concerned, and we shall find 

 in them the law of longitudinality beautifully carried out, the head is more clearly shown 

 to be the anterior representative of the pelvis. It is, of course, assumed that the verte- 

 bral theory of the skull is true, and that it also applies to the other or caudal extremity of 

 the vertebral column, where the physiological degradation is usually as great as is the 

 elevation anteriorly ; with the difference that the tendency to linear multiplication is 

 usually limited to four cranial vertebra}, while the tail may vary greatly in the number of 

 its aborted vertebral segments. The four regions of the body are associated physiologically 

 also in the same w T ay : the two extremes are regions of relation ; their functions, sexual and 

 mental, are exercised with direct reference to other individuals, and the latter is capable of 

 elevation to the highest communion possible between the finite and the infinite. But the 

 functions of the two intermediate regions are of a personal and selfish nature, those of the 

 abdomen being concerned in the support of the life of the individual within itself, and those 

 of the thorax evolving the power necessary for motion and for influence upon the world. 



The question next arises, Where is the longitudinal centre of the body ? This cannot be 

 indicated precisely, and perhaps varies in different species, but it undoubtedly lies between 

 the two intermediate regions, thorax and abdomen, perhaps at that vertebra whose spinous 

 process is upright, inclining neither backward like those of the dorsal, nor forward like 

 those of the lumbar vertebra? ; but this point will seem to shift its position according to 

 the various physiological requirements as to the length of the thorax or abdomen, or the 

 strength and mobility of the extremities. 



Questions of morphology are often determined by embryology ; — by reference to the 

 early stages of development before teleological modifications have been superadded ; the 

 development of the vertebral column and of the enclosed myelon, conclusively shows that 

 the morphological centre of the body is at the middle of the back, notwithstanding the 

 cephalic end afterward acquires such physiological superiority. Ossification of the vertebral 

 column commences at the middle of its length, and proceeds forward and backward from 

 this point: the size of the bodies, or centra, of the vertebra} at this early period, diminishes 

 anteriorly and posteriorly, and only later becomes more equal, which latter proportion 

 persists through life in the lower mammalia but in the quadrumana and especially in man 

 the posterior centra become thicker and stronger, so as to give the column that slender 

 pyramidal shape necessary for the preservation of their more or less erect position, while 

 the upper or neural arch is anteriorly expanded to accommodate the enormously enlarged 

 cerebral ganglia, the long axis of which forms a decided angle witli that of the cord. 



Perhaps nowhere is the distinction between morphology and teleology more evident than 

 in the cerebrospinal axis. At the first appearance of this in the embryo, there is no 



MEMOIRS DOST. SOC. NAT. HIST. VOL. I. 16 



