The haemal arch is absent in the human coccyx; but it exists in the tails of many animals, where it is composed of haemapophyses and a haemal spine, the former slightly displaced, so as to articulate like the dorsal pleurapophyses, or ribs, with the contiguous borders of two vertebrae and their intervening fibro-cartilage. In this canal runs the long caudal artery, protected by the bony arch from pressure, and giving off small transverse branches, the homotypes of the intercostals of the thorax, or abdomen, in its whole course.
In fish we notice a haemal as well as a neural arch attached to the trunk vertebrae; but it is formed, not by the haemapophyses, but by parapo-physes, which bend downwards, and, uniting infe-riorly, form by their succession a long haemal canal. The facility with which particular vertebral elements change their form or direction is one great reason why a new nomenclature has been introduced. It would be obviously a misapplication of terms to call these processes in the fish transverse when they curve downwards. Neither can they be called pleurapophyses, or ribs; for these latter bones are attached in the upper thoracic region to the distal extremities of the parapophyses, whose independency is thus established. Whilst engaged in the study of Homology the mind should never lose sight, in the first place, of the facility with which different vertebral elements may by some change of form be made subservient to the same functions; and in the second place, of the readiness with which the same element is converted into a variety of shapes, that it may effect some particular purpose in any of the different segments of the body.
The proper connections, and the typical signification of the bones called "ossa nominata," are obscured in man by their great expansion and early coalescence. They still, however, form a haemal arch in which are lodged the coccygeal blood-vessels, the termination of the intestinal tube, the bladder and urethra, and the organs of generation. We have already seen that part of the solid sacrum, external to the anterior sacral foramina, represents the coalesced pleurapophyses of the sacral vertebrae. What, then, is the meaning of the three elements, ilium, ischium, and pubes, the assemblage of which constitutes an os innominatum ?
Professor Owen, after shewing that the ilium, however expanded, belongs to one vertebral segment only, states that the haemal arch, of which it x forms a part, is open to two interpretations. The piece of bone confluent with the sacrum may be the whole pleurapophysis; the ilium, the haemapophy-sis, and the pubes, the half of an expanded and bifid haemal spine; or the piece confluent with the sacrum, together with the ilium, may be two portions of a teleologically compound pleurapophysis; and the pubes the haemapophysis, which would join with its fellow without or with a mere rudiment of a haemal spine intervening. From the analogy of the scapulo-coracoid arch in fishes, which is proved by its modifications in higher animals to want the haemal spine, it is most probable that such is the condition and true interpretation of the correspondingly simple pelvic arch under consideration.
The ischia are the haemapophyses of the second sacral vertebra.
In the turtle the iliac bones, elongated and of cylindrical form, are attached superiorly to an outstanding process from one sacral vertebra only; from their distal extremity the broad pubic bones take their rise, and meet at the symphysis to complete the haemal arch. The ossa ischii lie immediately posterior to the pubic bones, and the interval between the two constitutes the obturator formen, which may be divided into two by a process of bone, or by a ligament extending backwards from the pubic to the ischiatic symphysis. In man the ossa ischii diverge to give passage to the urethra, but in many of the lower vertebrate animals their union is as close as the pubic symphysis. No better illustra- , tion of the typical character of the pelvic bones can be found than in the arrangement of the iliac, pubic, and ischiatic bones in the mole. They there form a small haemal arch, not very dissimilar in its general form from the succession of arches which protect the coccygeal artery in the tail of the ferret or of the kangaroo.
Of the seventeen vertebrae which intervene between the cervical region and the sacrum, twelve only, named dorsal, are in man supplied with a complete haemal arch; the five inferior, named lumbar, have stunted pleurapophyses, which afford attachment by their extremities to the tendinous origin of the transversalis abdominis muscle. The completion of the haemal arch in the lumbar region by soft and compressible, not by hard and unyielding, structures, is needed, that the abdominal walls may adapt themselves to the movements of the diaphragm, and maintain just the proper degree of compression upon the abdominal viscera. In animals that are prone, in whom the abdominal viscera require a greater amount of support than in man, the elongated and movable pleurapophyses are proportionately more numerous. In the dorsal region, the long, flat, and curved pleurapophyses articulate, by their proximal extremities, with the contiguous borders of two vertebrae and their intervening fibro-cartilage; by their distal extremities with the cartilages of the ribs, or the unossified haemapophyses which unite with the chain of sternal bones or the haemal spines, to complete the haemal arch. The flat sternum of man, continued to the pubes by the tendinous linea alba, is represented, in the alligator, by a chain of bones which extend in front of the abdomen, and support both lumbar and dorsal haemapophyses. In birds there is the same rapid confluence of the different pieces composing the sternum as is observed in the development of the cranium; but from the mesial line there projects a long and usually prominent plate of bone, formed of the coalesced haemal spines, from either side of which arise the great pectoral muscles.
The haemal arch is imperfect in the cervical region. The diverging parapophyses and diapophy-ses coalesce with a short pleurapophysis, concave superiorly, in man, to support the spinal nerves as they emerge from the intervertebral foramina behind the vertebral artery. None of the organs contained in the neck require the protection of a bony arch, which would interfere with the mobility requisite for the varied movements of the head. The only haemapophyses typically belonging to the cervical vertebrae are the clavicles, which, displaced from the atlas, extend from the first bone of the sternum outwards to the equally displaced scapulo-coracoid arch of the occiput. The proper signification of the clavicles is seen in the furcular bone of the echidna, or of birds, where it forms a haemal arch more complete than is effected by the distinct and separate clavicles of man.
Nowhere is the importance of homology more clearly exemplified that in the study of the attachments of muscles; and perhaps, in illustration of the truth of this statement, we may cite the muscles of the back, whose origins and insertions, as commonly described, may be considered as forming one of the most complicated and most unsatisfactory portions of Human Anatomy. The muscles of the back are either longitudinal or oblique; that is, they either pass vertically downwards from spinous process to spinous process, from diapophysis to diapo-physis, from rib to rib (pleurapophyses), etc.; or, they extend obliquely from diapophysis to spine ; or, from diapophysis to pleurapophysis, etc.