Part 4 (1/2)

In the human skeleton, the internal lip of the trochlea descends lower than the external; and also lower than the condyle In the bear, the cat, and the dog, it is the same In the ox and the sheep, the condyle is lower than the trochlea, but only very little lower In the horse the arrangement is still the same, but a little more accentuated

On the lateral parts of this extremity we find: internally, a prominence, the epitrochlea; and, externally, another, the epicondyle

It is fro upwards, forroove of torsion

The two proeneral point of view, present special arrangements which it is necessary to point out When we examine the form of the outline of the inferior extre, the ox, and the horse, we find in following this order that the extremity tends to become narrow transversely, and that the epicondyle and the epitrochlea are less and less prominent on the external and internal aspects respectively These two processes, indeed, project backwards; the epitrochlea always re more developed than the epicondyle Because of this projection backwards, the cavity situated on the posterior surface of the inferior extremity, the olecranon fossa, is very deep,thus formed by the two processes, are very prominent In front we find the coronoid fossa, which is less deep than that of which we have just spoken

There exists in some mammals an osseous canal, situated above the epitrochlea, and known as the _supratrochlear canal_ (Fig 19) It is bounded by a plate of bone which at its middle portion is detached from the shaft of the humerus, and blends with the latter at both its extreh the foramen

[Illustration: FIG 19--INFERIOR EXTREMITY OF THE LEFT HUMERUS OF A FELIDE (LION)

1, Epitrochlea; 2, supra-epitrochlear foramen]

[Illustration: FIG 20--INFERIOR EXTREMITY OF THE LEFT HUMAN HUMERUS, SHOWING THE PRESENCE OF A SUPRATROCHLEAR PROCESS

1, Epitrochlea; 2, supra-epitrochlear process]

A similar condition is sometimes found, as an abnor aspect (Fig

20): an osseous pro, in the shape of a crochet-needle--supra-epitrochlear process--situated 5 or 6 centiives attachment to a fibrous band, which is inserted by its other end into the epitrochlea and the internal interives passage to the brachial artery and the median nerve, or in case of a premature division of this artery to the ulnar branch of the same[9]

[9] For further details of this anomaly, see Testut, 'The Epitrochlear Process in Man' (_International Journal of Anatoy_, 1889); A Nicolas, 'New Studies on the Supratrochlear Process in Man' (_Review of Biology of the North of France_, t

iii, 1890-1891)

There is also found in some mammals a perforation of the thin plate of bone which, in others, separates the olecranon fossa from the coronoid

This perforation is sometimes found as an abnormality in the human humerus

As does the sternum and the skeleton of the shoulder, the humerus of birds presents differences correlated to the functions which the thoracic li on the side of the thorax, directed obliquely doards and backwards (Fig 21), it is proportionately longer in individuals of powerful flight than in those which fly less or not at all In the vulture it projects beyond the posterior part of the pelvis; in the cock it does not even reach the anterior border of the sath are added differences in volume and in the development of the processes which serve for muscular attachht

The humerus is so placed that the radial border, external in man and quadrupeds, looks upwards, with the result that the surface of the bone of the arm, which in these latter is anterior, in the former looks outwards The humeral head, which is turned forwards and a little inwards, is convex and elongated in the vertical direction Behind and above this head is found a crest for the insertion of ion belohere there is a tuberosity whose inferior surface presents a pretty large opening which looks inwards to a fossa fros communicate with the interior of the bone This is the pneumatic foramen of the humerus

It is of interest to reme with the conditions of flight, every systeht of the body We particularly draw attention to the osseous fraht of the anireatly lessened This condition is secured by the pneumaticity The bone consists of a cover of co marrow, is hollowed out by cavities which contain air, and communicate with special pouches, the air-sacs, which are appendages of the lungs[10]

[10] The presence of air in the bones does not seeht; as a matter of fact, we find air spaces in the bones of soht of Birds,' Paris, 1890, p 51)

[Illustration: FIG 21--SKELETON OF A BIRD (VULTURE): LEFT SURFACE

1, Cranium; 2, face; 3, cervical vertebrae; 4, spinous processes of the dorsal vertebrae; 5, coccygeal vertebrae; 6, sternum; 7, keel; 8, superior ribs; 9, inferior ribs; 10, clavicle; 11, coracoid bone (for the details of the skeleton of the shoulder, see Fig 18); 12, humerus; 13, radius; 14, ulna; 15, carpus; 16, hand (for details of the skeleton of this region, see Fig 31); 17, ilium; 18, ischiu 46); 20, femur; 21, tibia; 22, fibula; 23, osseous nodule, which some anatoe of the tarsus; 24, metatarsus; 25, foot; 26, first toe (for the details of the skeleton of the foot, see Fig 48)]

The antibrachial extremity of the humerus is flattened from without inwards It terminates in two articular surfaces, which articulate with the radius and ulna

The olecranon process of the ulna being slightly developed, it follows that the olecranon fossa is not large; neither is the coronoid