246 OSTEOLOGY. 
occasionally be met with. The frequent occurrence of an increase in the number of digits seems 
to indicate that phylogenetically the number of digits was greater than at present, and included a 
prepollex or prehallux, and a digitus post-minimus. The correspondence of the metacarpus with 
the metatarsus and the phalanges of the fingers with those of the toes is s0 obvious, that it is 
sufficient merely to mention it. 
The differences in size, form, and disposition of the skeletal elements of the hand and foot is 
easily accounted for by a reference to the functions they subserve. 
In the hand strength is sacrificed to mobility, thus leading to a reduction in the size of the 
carpal elements, and a marked increase in the length of the digital phalanges. The freedom of 
movement of the thumb, and its opposability to the other digits, greatly enhances the value of 
the hand as a grasping organ. In the foot, where stability is the main requirement, the tarsus 
is of much greater proportionate size, whilst the phalanges are correspondingly reduced. Since 
the foot no longer serves as a grasping organ, the great toe is not free and opposable like the 
thumb. 
Limb Girdles.—The free limbs are linked to the axial skeleton by a chain of bones 
which constitute their girdles. The fundamental form of these limb girdles consists each of a 
pair of curved cartilages placed at right angles to the axis of the trunk, and embedded within its 
musculature. Each cartilage has an articular surface externally about the middle for the recep- 
tion of the cartilage of the first segment of the free limb. In this way each pectoral and pelvie 
cartilage is divided into an upper or dorsal half and a lower or ventral half. The dorsal halves 
constitute the scapula and ilium of the pectoral and pelvic girdles respectively. With regard to 
the ventral halves, there is more difficulty in establishing their homologies. The original con- 
dition is best displayed in the pelvic girdle ; here the ventral segment divides into two branches— 
one anterior, which represents the pubis, the other posterior, which ultimately forms the ischium. 
Ventrally, the extremities of these cartilages unite to enclose the obturator foramen. In the 
pectoral girdle the disposition of the ventral cartilages is not so clear, consisting primitively of 
an anterior branch or precoracoid, and a posterior portion or coracoid ; these, in higher forms, 
have undergone great modifications in adaptation to the requirements of the fore limbs. The 
posterior or coracoid element, the homologue of the ischial cartilage in the pelvic girdle, is but 
feebly represented in man by the coracoid process and the coraco-clavicular ligament. With 
regard to the homologue of the pubic element in the pectoral girdle, there is much difference of 
opinion ; in reptiles and amphibia it corresponds most closely to the precoracoid, but it is doubt- 
ful what represents it in mammals. According to Goette and Hoffman, the clavicle is a primor- 
dial bone, and not, as suggested by Gegenbaur, of secondary or dermic origin. If this be so, it 
corresponds to the ventral anterior segment of the pectoral girdle, and is therefore homologous 
with the ventral anterior (pubic) segment of the pelvic girdle. On the other hand, if Gegen- 
baur’s view be accepted, the clavicle has no representative in the pelvic girdle. It must, how- 
ever, be borne in mind that during its ossification it is intimately associated with cartilage, and 
that that cartilage may represent the precoracoid bar ; nor must too great stress be laid upon the 
fact that the clavicle begins to ossify before it is preformed in cartilage, since that may be merely 
a modification in its histogenetic development. 
According to another view (Sabatier), the subcoracoid centre (see Ossification of Scapula) is 
derived from the posterior ventral segment, and corresponds to the ischium, whilst the coracoid 
process is the remains of the anterior ventral segment (precoracoid), and is homodynamous with 
the pubis. 
In no part of the skeleton does function react so much on structure as in the arrangement of 
the constituent parts of the pectoral or pelvic girdles. In man, owing to the assumption of the 
erect position and the bipedal mode of progression, the pelvic girdle acquires those characteristics 
which are essentially human, viz. its great relative breadth and the expansion of its iliac por- 
tions, which serve as a support to the abdominal viscera, and also furnish an extensive origin to 
the powerful muscles which control the movements of the hip-joint. The stability of the pelvie 
girdle is ensured by the nature of its union with the axial skeleton, as well as by the osseous 
fusion of its several parts, and their union in front at the symphysis pubis. 
In man, since the erection of the figure no longer necessitates the use of the fore limb 
as a means of support, the shoulder girdle has become modified along lines which enhance its 
mobility and determine its utility, in association with a prehensile limb. Some of its parts 
remain independent (clavicle and scapula), and are united by diarthrodial joints, whilst others 
have become much reduced in size or suppressed (coracoid-precoracoid, see ante). The dorsal 
part of the girdle (scapula) is not directly united with the axial skeleton as is the ilium, but is 
only indirectly joined to it through the medium of the clavicle, which is linked in front with the 
presternum. The same underlying principles determine the differences in mobility and strength 
between the shoulder, elbow, and wrist, and the hip, knee, and ankle joints of the fore and hind 
limbs respectively, whilst the utility of the hand is further enhanced by the movements of pro- 
nation and supination which occur between the bones of the forearm. In the leg such move- 
ments are absent, as they would interfere with the stability of the limb. 
a! 
