188 OSTEOLOGY. 
humerus articulates with the great sigmoid cavity of the ulna. On the anterior 
surface of the bone, immediately above the trochlea, is a depression—the coronoid 
fossa (fossa coronoidea), in which the coronoid process of the ulna rests in flexion 
of the joint, whilst in a corresponding position on the back of the lower end of 
the shaft there is a hollow, called the olecranon fossa (fossa olecrani), just above the 
trochlea posteriorly. Into this the olecranon process sinks when the elbow is 
extended. The two fossz are separated by a thin translucent layer of bone which 
may be deficient, thus leading to the formation of a foramen between the two 
hollows in the macerated bone. The anterior part of the capsule of the elbow 
joint is attached to the superior margins of the radial and coronoid fossee in 
front, whilst the posterior ligament is connected with the upper border and lateral 
edges of the olecranon fossa behind. The strong internal and external lateral 
ligaments are attached superiorly to the internal and external epicondyles respec- 
tively. The proportionate length of the humerus to the body height is as 1 is to 
29 3*5:25. 
Nutrient foramina are usually to be seen, one at or near the surface for the insertion of the 
coraco-brachialis, the other usually close to the hinder border of the deltoid eminence ; both 
have a downward direction. Numerous vascular foramina are scattered along the line of the 
anatomical neck, the larger ones being situated near the upper end of the bicipital groove. 
Connexions.—The humerus articulates with the scapula above, and radius and ulna below. 
Embedded as the humerus is in the substance of the upper arm, its shaft and head are surrounded 
on all sides. It is only at its lower part that it comes into direct relation with the surface, the 
internal epicondyle forming a characteristic projection on the inner side of the elbow ; whilst the 
external epicondyle, less prominent, and the external epicondylic ridge can best be recognised 
when the elbow is bent. 
Architecture.—The shaft consists of a layer of compact bone surrounding a long medullary 
canal. The outer shell, thickest in the lower third of the bone, gradually thins until it reaches 
the superior epiphysial line, where it forms a layer no thicker than stout paper. Inferiorly 
the external shell is thicker and stouter than above, until it reaches the epicondyles, below 
which the articular surfaces are formed of a layer of compact spongy bone. The upper end of 
the medullary canal is surrounded by loose spongy tissue, the fibres of which arch inwards 
from the inner surface of the compact outer layer, whilst inferiorly the cancellous tissue 
which springs from the outer shell sweeps downwards in a radiating fashion on either side 
of the olecranon fossa towards the epicondyles. Above the olecranon fossa there are a 
number of laminze of dense bone which arch across from one side to the other, the con- 
vexity of the arches being directed downwards. The superior epiphysis, formed of spongy 
bone, is united to the shaft by a wavy line, concave externally and convex internally, leading 
from the base of the great tuberosity on the outer side to the inferior articular,edge on the inner 
side. The mass above this includes the head and two tuberosities. The spongy tissue of the 
head is fine, and is arranged generally in lines radial to its surface ; that of the great tuberosity 
is more open, and often displays large spaces towards its interior, which in old bones communi- 
eate freely with the medullary cavity of the shaft. The general direction of the fibres is 
parallel to the outer surface of the tuberosity. The lower articular end is formed of fine spongy 
tissue, more compact towards the surface, and arranged in lines more or less at right angles to its 
articular planes. In the adult the principal nutrient canal, viz. that which opens on the 
surface near the insertion of the coraco-brachialis, traverses the outer compact wall of the shaft 
obliquely downwards for a distance of two and a quarter inches before it opens into the 
medullary cavity. 
Variations.—As has been already stated, the oleeranon and coronoid fossve may communicate 
with each other in the macerated bone. The resulting supratrochlear foramen is more 
commonly met with in the lower races of man, as well as in the anthropoid apes, and in 
some other mammals. The occurrence of a hook -like spine, called the epicondylic process, 
which projects in front of the internal epicondylic ridge, is not uncommon. Its extremity is 
connected with the internal epicondyle by means of a fibrous band, underneath which the median 
nerve, accompanied by the brachial artery, or one of its large branches, may pass, or in some 
instances, the nerve alone, or the artery unaccompanied by the nerve. This process is the homo- 
logue in a rudimentary form of a canal present im many animals, notably in the carnivora and 
marsupials. In addition to the broad musculo-spiral groove already described, and which is no 
doubt produced by the twisting or torsion of the shaft, there is occasionally a distinct narrow 
eroove posterior to it, which marks precisely the course of the musculo-spiral nerve as it turns 
round the outer side of the shaft of the bone. 
Ossification.—At birth the shaft of the humerus is usually the only part of the 
bone ossified, if we except the occasional presence (22 per cent) of an ossific centre in the 
head. (H. R. Spencer, Journ, Anat. and Physiol. vol. xxv. p. 552.) The centre for the 
shaft makes its appearance early in the second month of intrauterine life. Within the 
first six months after birth a centre usually appears for the head ; this is succeeded by 
one for the great tuberosity during the second or third year. These soon coalesce ; and 
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