312 THE HORSE. 
thorax is really occupied by the abdominal organs. This will be hereafter 
more fully explained in examining these parts at page 407, e¢ seq. 
THE PELVIC ARCH AND HIND EXTREMITIES. 
BEHIND THE THORAX occurs a second interval corresponding to the 
loins, where the hemal arch is deficient ; but at the pelvis the circle is 
completed by the bones of the ischium, ilium, and pubes, united to the 
sacrum above, and having the hind extremities firmly articulated to them 
at the hip joints. The pelvis constitutes not only a firm and solid case for 
the protection of the large blood-vessels, and of the urinary and genital 
organs, but it is also intimately connected with locomotion, to which the 
posterior extremities largely contribute. 
EE RAL 
THIS ORGAN appears to be intended chiefly to protect the body from 
insects ; but it also serves to some extent as an aid in balancing the body 
when rapidly moving in any new direction. It is made up of from fifteen 
to eighteen bones, which will be described in the next chapter. 
THE FORE AND HIND EXTREMITIES CONSIDERED AS 
ORGANS OF LOCOMOTION. 
THE SEVERAL BONES which are connected together to form the ex- 
tremities must be regarded, first, as organs of support, and, secondly, as 
the primary means of locomotion. Each extremity consists of correspond- 
ing divisions, the ilium being the analogue of the scapula; the femur of 
the humerus ; the tibia and fibula of the ulna and radius; the tarsus of 
the carpus; and, lower down, the bones of the fore and hind extremity 
being precisely similar in their forms. There is, however, a want of 
exact correspondence in the form and direction of the joints, which has 
been much insisted on by homologists, and which Mr. Maclise has 
attempted to explain in his very interesting article on “The Skeleton,” 
contained in the ‘‘ Cyclopedia of Anatomy and Physiology” edited by 
Dr. Todd. It appears to me that this does not make matters more 
simple, but the reverse, and that the proper point of view is to regard the 
humerus and femur as homologous, and the tibia, with the patella, as ana- 
logous to the ulna and its olecranon process. The intervention of the 
ligament between the patella and tibia does not affect the use of the 
former as a lever; and both joints being perfectly hinge-like in their 
actions, there is no other important difference. Descending to the next 
joint, the os calcis stands out as an important lever, and is represented in 
the carpus by the os pisiforme (called by Percivall os trapezittm), each 
having important muscles inserted in their upper edges. It is true that 
the one is an agent in what is called extension, while the other is engaged 
in flexing the limb ; but this is only dependent upon the limit to motion 
in either direction. Below the carpus and tarsus there is no necessity for 
continuing the comparison. 
REGARDING THE LIMBS AS MEANS OF SUPPORT, it must be remembered 
that the fore limbs are nearer the centre of gravity, and, therefore, sustain 
more weight than the hind. The fore quarter is suspended between the 
bases of the two shoulder-blades, chiefly by the serrati magni, and in such 
@ Way as to require no special muscular contraction, The four parts of 
