The discovery and handling of the Harper fragment is complex and intriguing but beyond the scope of this paper. The HSCA provides a short summary:
(105) On the day after the assassination, at about 5:30 p.m. William Allen Harper, a student at Texas Christian University, was taking photographs of the Dealey Plaza area when he discovered a piece on bone near the scene of the assassination. Harper informed the FBI that he took the bone to his uncle, Dr. Jack C. Harper, and that they both then went to Dr. A. B. Cairns, chief of pathology at Methodist Hospital, Dallas Tex. Dr. Cairns believed the bone to be a piece of human skull. William Harper said he then gave the specimen to Special Agent Anderson of the FBI on November 25. 
This summary is accurate but incomplete and potentially misleading. Dr. Cairns was more specific in his identification of the bone fragment; he identified the fragment as probably occipital bone.  The HSCA characterized the fragment as parietal bone based largely on statements by Dr. J. Lawrence Angel, Curator of Physical Anthropology for the Smithsonian Institution. 
Since the debate over the origin of the fragment has important implications
for the authenticity of the medical evidence, the first issue is if the
photographs Dr. Cairns took of the fragment are the same as the photographs
examined by the HSCA. Mary Ferrell provided the author with copies of the
photographs made by Dr. Cairns (one showing the outer surface of the
fragment and the other showing the inner surface); one of these photographs
is reproduced in Figure 1 and compared to the photograph examined by the
HSCA. Even at low magnification, there is no doubt that the photographs
depict the same fragment.
|Figure 1. The Harper Fragment. A. Photograph of the Harper fragment published by the House Select Committee on Assassinations. B. Copy of photograph of the Harper fragment made by Dr. A. B. Cairns. C. Comparison of some anatomical features of A and B. Parietal foramina and vascular grooves are visible on both photographs and the contours of the fragments are identical.|
The second issue is if the fragment is occipital or parietal bone. Answering this question is relatively easy to a neuroanatomist but involves highly technical arguments and terms.
Dr. J. Lawrence Angel described the fragment in a memorandum addressed to the HSCA:
The Harper fragment photographs show it as a roughly trapezoidal piece, 7 centimeters by 5.5 centimeters in size, coming mainly from the upper middle third of the right parietal bone. Near its short upper edge vascular foramina on the inside and a faint irregular line on the outside indicate saggital suture. Its posterior inferior pointed edge appears to fit the crack in the posterior section of the right parietal [bone] and its slightly wavy lower border can fit the upper edge of the loose lower section of right parietal [bone]. Its upper short border, on the left of the midline near vertex, may meet the left margin of the gap. Behind it there appears to be a large gap and in front a narrow one.
Dr. Angel's placement of the Harper fragment is shown in Figure 2, which
also provides a rough guide for the distinction between parietal and
occipital regions of the skull.
|Figure 2. Location of the Harper Fragment. The location of parietal and occipital regions is shown on the drawing of John Kennedy's skull made by the HSCA. Dr. Angel's placement of the fragment is illustrated in the upper right figure with a magnified view illustrated just below his drawing. Gross anatomical features of the internal aspects of occipital and parietal bone are illustrated in the lower portion of the figure (adapted from Gray's Anatomy, 35th British Ed., edited by R. Warwick and P.L. Williams, Philadelphia: W.B. Saunders, 1973).|
The anatomical features of the Harper fragment demonstrate conclusively that it cannot be occipital bone. Some of the anatomical features that establish this are described below; these should be compared to Figure 2 which illustrates the anatomical features of parietal and occipital bone and illustrates the location of some features on the fragment.
(1) Vascular grooves
The inner surface of the skull is marked in places by vascular grooves, i.e., small depressions where blood vessels are located in vivo. In the case of parietal bone, vascular grooves are mainly from branches of the middle meningeal. No such pattern exists for occipital bone; it has an entirely different type of interior surface which will be described below. The photograph of the interior surface of the Harper fragment (HSCA Fig. 27; see Figure 1A and compare to Figure 1C) shows a pattern of vascular grooving entirely consistent with it being parietal bone and entirely inconsistent with it being occipital bone.
In contrast to parietal bone, occipital bone does not show a pattern of vascular grooving. It does have internal markings, including deep sulci ("grooves") that are much larger than vascular grooves; these are grooves for the transverse sinus and superior sagittal sinus. No such deep grooves are visible in the photographs of the Harper fragment.
(2) Additional features
Parietal bone is characterized by a relatively smooth (excluding vascular grooves) inner surface, mild curvature, and relatively uniform thickness. In contrast, occipital bone is characterized by major variations on its internal surface (i.e., many different bumps and grooves from various things), much greater curvature, and substantial variation in thickness (compare drawings of internal aspects of parietal and occipital bone in Figure 2). Simply put, occipital bone doesn't look like the fragment in Figure 1 but parietal bone does. There are numerous other reasons why the Harper fragment is parietal bone. For example, parietal foramina (vascular perforations of a type that occur only in parietal bone) visible in the photograph establish the location and orientation of the fragment. It is worth mentioning that if the Harper fragment were lower occipital bone, death would have been virtually instantaneous. The lower portion of occipital bone forms the foramen magnum (the space through which the forebrain connects to the spinal cord); for numerous reasons, it is virtually inconceivable that John Kennedy would have shown any vital signs following explosive destruction of this area.
The information reported here establishes that the Harper fragment is parietal, not occipital bone. This fact should not be over-interpreted. The conclusion supports the authenticity of the medical evidence, but does not prove it. More importantly, the origin of the Harper fragment as parietal bone does not in any way support the conclusion that John Kennedy was struck in the head by one and only one bullet. The conclusion simply clarifies the remaining issues in evaluating the medical evidence.
The controversy over the autopsy of John Kennedy has generated many unresolved questions about the medical evidence. However, the available evidence is sufficient to determine the origin of the Harper fragment based on the anatomical features of the fragment. These anatomical features no doubt seem obscure to the general reader but they are definitive to a neuroanatomist. All of the features of the Harper fragment are consistent with it being parietal bone and inconsistent with it being occipital bone; there can be no reasonable scientific doubt that the Harper fragment is parietal bone.
The author thanks Mary Ferrell for sharing the photographs of the Harper fragment and for her tireless dedication and inspiration.