140 MOBPHOLOGY OF THE BONES OF THE LIMBS. 



In the infant the head of the astragalus is directed more inwards than in the adult, and 

 the foot is naturally inverted. The first metatarsal bone is also relatively short in the foetus, 

 and at first is inclined inwards from the internal cuneiform bone, the distal articular surface 

 of which is oblique. In these respects the young foot is more like that of the ape. (Leboucq , 

 " Le developpement du premier metatarsien et de son articulation tarsienne chez I'homme," 

 Arch, de Biologie, iii, 335.) 



MOEPHOLOGY OF THE BONES OF THE LIMBS. 



Relation to the axial skeleton. Anatomists have generally agreed to look upon the relation 

 which the bones of the limbs bear to the rest of the skeleton as that of appendages to the 

 trunk, and most are also disposed to regard these appendages as similar radiations or extensions 

 from one or more of the vertebral segments in two determinate situations of the trunk. But 

 opinions are much divided as to the typical number of the vertebral segments which are in- 

 volved, and as to the exact morphological nature of the parts which form the radiations. The 

 quinquifid division of the peripheral parts of both limbs in man and many animals appears 

 favourable to the view that each has the elements of five segments prolonged into it ; but of 

 late years attention has been specially directed by K. Bardeleben and others to certain, mar- 

 ginal structures which are interpreted as vestiges of more or less completely suppressed digits, 

 and there seems reason for believing that the pentadactyle extremity has been preceded by a 

 heptadactyle form. In association with this it may be noticed also that the nerves entering 

 into the proper limb-plexuses are in each case seven, viz., the 4th, 5th, 6th, 7th and 8th 

 cervical, with the 1st and 2nd dorsal, in the brachial plexus, and the 2nd, 3rd? 4th and 5th 

 lumbar, with the 1st, 2nd and 3rd sacral in the crural plexus. It is farther generally held 

 that the constituent elements follow each other in a similar order in the two limbs from the 

 cephalic to the caudal part of the vertebral axis, so that the pollex and radial elements occupy 

 the cephalic side of the upper, while the hallux and tibia take the same place in the lower 

 limb. 



Homological comparison of upper and lower limbs. A certain anatomical correspond- 

 ence between the upper and lower limbs, which is apparent to common observation, is 

 admitted in even a fuller degree by most anatomists as a result of a careful comparison of the 

 form and relations of their bones, as well as of their other parts, in both their embryonic and 

 fully formed conditions ; and the general conclusion has been formed that the thoracic and 

 pelvic limbs are constructed on the same general type in man and animals, both as regards 

 the attaching girdles of the shoulder and pelvis, and in the three several sections of which 

 each limb is composed. There are, however, certain modifications of that general plan, 

 leading to considerable differences in the form, size, and number of the individual parts in 

 different animals, which appear to be in a great measure related to the different uses to which 

 the upper and lower limbs are respectively applied ; as, for example, in the upper limb of 

 man, the breadth of the shoulders, caused by the interposition of the clavicle, the greater 

 extent of motion in the shoulder- joint, the eversion of the humerus, and the forward flexed 

 attitude of the elbow-joint, the arrangements for pronation and supination by rotation of the 

 radius and hand, and the opposability of the thumb, all have reference to the freedom, 

 versatility, and precision of the movements of the upper limb as an organ of prehension and 

 touch ; while in the lower limb, the comparatively fixed condition and arched form of the 

 pelvic girdle, the greater strength of the bones, the close-fitting of the hip- joint, the inversion 

 of the femur, the backward flexure of the knee-joint, the arched form of the foot, and non- 

 opposability of the great toe, have all manifest relation to the support of the trunk and 

 pelvis, and their movements upon the lower limbs. In the lower animals, greater modifica- 

 tions in the form of both limbs are to be observed, obviously adapted to their different 

 functions in each case. 



Without attempting to follow out this subject by any detailed reference to comparative 

 anatomy or development, it may be useful to state here shortly the more probable conclusions 

 which have been formed with respect to the homological correspondence of the several parts 

 of the upper and lower limbs. 



Shoulder and pelvic girdles. In each of these a division is to be recognized at the place 

 of attachment of the limb-stalk (shoulder and hip-joints) into a dorsal and a ventral section. 

 The dorsal section in the upper limb is the scapula, in the lower limb the ilium, which 

 accordingly correspond, the chief difference between them consisting in the scapula being 

 free from articulation with the vertebral column, and therefore capable of a considerable 

 degree of motion, while the ilium is firmly jointed to the lateral mass of the sacrum. The 

 ventral section is in each case double, including the clavicle and coracoid in the shoulder- 

 girdle, and the pubis and ischium in the pelvic girdle. It is generally admitted that the 

 coracoid and ischium are corresponding structures, the coracoid being reduced to a relatively 



