116 BONES OF THE LIMBS. 



same side of the body. This latter hypothesis must be regarded as particularly 

 unhappy, as it is difficult to conceive how any substantial homology, or a resem- 

 blance other than fanciful, can be traced by imagining half of a bone to correspond 

 with one structure, and the other half with a structure totally different. Equally 

 artificial is the proposition of Vicq D'Azyr to compare the upper limb of one side 

 with the lower limb of the other side of the body. The assumed correspondence 

 of the olecranon and patella, which has led to those theories, is not borne out by a 

 comparison of the development of the two structures. The patella is formed, distinct 

 from the tibia, as a sesamoid bone in a tendon, while the olecranon is mainly derived 

 from the principal centre of ossification of the ulna, and is formed only in part by the 

 superior epiphysis of that bone. In support of the more probable view, that the ulna 

 corresponds with the fibula, it is to be noticed that in the echidna and some other 

 animals the head of the fibula is prolonged upwards into a process which much more 

 closely corresponds with the olecranon than does the patella. It may also be observed 

 that in certain mammals, such as the bear or other plantigrades, in which there is a 

 power of partial pronation, the radius crosses the forearm obliquely, and its upper 

 end is brought somewhat in front of the ulna; while in the greater number of 

 mammals, in whi'ch there is no power of pronation or supination, the radius is 

 placed entirely to the front and internally, and the ulna is thrown to the outside 

 and behind, in the same manner as the fibula is to the outside of and behind the 

 tibia ; and the resemblance between the respective bones in the fore and hind 

 limbs is made the more striking that the radius and tibia are in these animals 

 generally the principal bones, while the ulna and fibula are in many species only 

 partially developed. An examination of the fore limb in a series of animals shows 

 that the articulation of the radius, as in man, with a separate portion of the 

 humerus external to that with which the ulna articulates is quite exceptional, the 

 most common arrangement being that the ulna forms the posterior and the radius 

 the anterior part of one great sigmoid cavity similar to that formed by the ulna in 

 the human subject. In comparing the humeral with the femoral region it may be 

 well to have regard to the apparent twist inwards which is seen in both femur and 

 humerus, and especially in the latter bone. This appearance of twisting is given 

 to the humerus by the direction of the musculo-spiral groove, and by the obliquity 

 in the direction of all the ridges of the bone. In the femur the twisted appearance 

 is very slight, and is most obvious below and in front of the small trochanter. In 

 the humerus the appearance of twisting is much more marked, and is more especially 

 obvious at the spiral groove. If, while the forearm remains unmoved, the lower end of 

 the humerus were turned outwards a quarter of a circle, so as to undo the twist of the 

 bone, the inner condyle would then overhang the flexor aspect of the forearm, and the 

 outer condyle the extensor aspect, and the flexor and extensor muscles would pass 

 directly downwards from the condyles to their terminations. Assuming this mode of 

 viewing the position of the humerus to be correct, we may proceed to compare the limbs 

 by considering the hand and foot, and also the forearm and leg, as having their flexor 

 surfaces directed towards the mesial plane of the body (the position in which they 

 are developed), while the condyles of the humerus and femur continue to be external 

 and internal in position, and the anterior or flexor surface of the humerus corresponds 

 with the anterior or extensor surface of the femur. From this it follows that one of 

 the propositions maintained by those who regard the tibia and ulna as homologous, 

 viz., that the quadriceps extensor femoris obviously corresponds with the triceps 

 brachialis, must be erroneous, and that the biceps femoris may more justly be viewed 

 as corresponding with the triceps brachialis, while the rectus femoris is homologous 

 with the biceps of the arm, and the vasti and crureus with the brachialis anticus 

 muscle, which in some animals extends up to the neck of the humerus ; so also the 

 space between the lips of the linea aspera of the femur (which in most animals is 

 much broader than in man) will correspond with th posterior surface of the humerus 

 below the musculo-spiral groove. 



The detailed comparison of the bones of the shoulder and pelvis is beset with 

 many difficulties, and it must therefore suffice here to remark that it requires further 

 investigation than has been brought to bear upon it, and simply to state that while 

 the scapula obviously corresponds with the ilium, the clavicle is generally regarded 

 as corresponding with the os pubis, and the coracoid process (or coracoid bone of 

 birds and reptiles) with the ischium. Humphry, indeed, has put forward the idea 



