THE HEAD. 211 
behind by ligamentous tissues (ligamenta subflava), which, opposite each 
intervertebral space, are pierced by openings on each side to give exit 
to the vertebral nerves passing out to the exterior of the body and to the 
extremities. Opposite to these openings the bone is notched above and 
below, ana these entervertebral notches complete the parts common to the 
whole serics. Thus the vertebral or spinal column serves as a firmly 
secured but flexible tube for the lodgment of the spinal cord, while at the 
same time it gives passage to its nerves. By this formation it is far less 
liable to injury, and also more useful as an aid to locomotion, than if it 
were made of one solid piece of bone, which, from its length, would be 
readily broken. 
OF THE HEAD AND FACE, AND OF THE HYOID ARCH. 
Mopern anatomists, following out the idea first suggested by Maclise 
and Owen, consider the head as made up of six vertebrae ; the posterior 
one, or that nearest to the neck, being the occipital bone, the next two 
being made up of the temporal bone, and the ultimate vertebree consisting 
of the sphenoid and «thmoid bones. This is a somewhat fanciful hypo- 
thesis, when worked out in detail; but it is obvious that the several 
bones of the skull subserve the same purposes as the vertebrae, and 
resemble those parts of the skeleton in forming a series of irregular 
arches to protect the brain, the division into separate pieces being far 
more secure than if the whole were in one. 
THE BONES OF THE FACE, including the lower jaw and os _ hyoides, 
depend from the neural arch or brain-case much in the same way as the 
ribs and pelvic bones posterior to them are attached to the vertebra, and 
though they inclose organs of less vital importance, yet they are perfectly 
analogous to these parts in their types and in the offices which they 
perform. 
OF THE THORACIC ARCH AND ANTERIOR EXTREMITIES. 
LyING IN THE HORSE AT SOME DISTANCE POSTERIORLY to the three first 
segments of the hamal arch (the bones of the face, lower jaw, and os 
hyoides), and separated from them by the neck, where there is a hiatus, 
the thoracic arch and anterior extremities depend from the vertebrze corre- 
sponding to them. In many of the higher vertebrates the fore extremity 
is firmly united by a joint to the thorax, and may be considered with it ; 
but in the horse it is only attached by muscles, the thorax being slung 
between the upper edges of the blade-bones by means of two broad sheets 
of muscular fibres. Hence the collar-bone is entirely absent in this 
animal ; and thus, while he is free from dislocations and fractures of that 
bone, to®which he would be constantly subject if it were present, he is 
rendered more liable to strains and rheumatic inflammations of the mus- 
cular sling, by which freedom of action is impaired. 
IN THE ARTICULATED SKELETON it is usual to consider the thorax as 
made up by the eighteen dorsal vertebree superiorly, the eighteen ribs 
and their cartilages on each side, and the sternum with its cartilages 
below. but the cavity of the thorax, as bounded by the diaphragm 
posteriorly, is not nearly so large as would be supposed from a considera- 
tion of the dry skeleton, for though the diaphragm is attached to the 
twelve posterior ribs near their cartilages, yet its surface is so convex 
towards the thoracic cavity, that a very large space within the bony 
