Domestic dogs exhibit an extraordinary degree of morphological diversity. Such breed-to-breed variability applies equally to the canine skull, however little is known about whether this translates to systematic differences in cerebral organization. By looking at the paramedian sagittal magnetic resonance image slice of canine brains across a range of animals with different skull shapes (N = 13), we found that the relative reduction in skull length compared to width (measured by Cephalic Index) was significantly correlated to a progressive ventral pitching of the primary longitudinal brain axis (r = 0.83), as well as with a ventral shift in the position of the olfactory lobe (r = 0.81). Furthermore, these findings were independent of estimated brain size or body weight. Since brachycephaly has arisen from generations of highly selective breeding, this study suggests that the remarkable diversity in domesticated dogs' body shape and size appears to also have led to human-induced adaptations in the organization of the canine brain.

Current research in our laboratory on the efferent discharges of the phrenic nerve (Landau and Akert, '60) suffered from a serious lack of information on the following subjects: (1) Number and diameters of motor fibers of the phrenic nerve with special reference to the A y component. ( 2 ) Topographical arrangement of the phrenic myotomes in the diaphragm. The present investigation was carried out with the hope and intention of closing this gap. MATERIALS AND METHODSSurgery. Dogs treated with sodium pentobarbital were used both in acute and chronic experiments. Surgical procedures were used for the exposure of the C5-C7 branches of the phrenic nerve as well as for the unilateral chronic de-afferentation of the phrenic nerve. The topographical situation at the origin of the phrenic nerve ( fig. 4 ) offers easy accessibility of all branches concerned with the possible exception of the branch from the C7 spinal nerve root. The latter is found between the subclavian artery and the first rib forming a small and relatively short subsidiary to the main trunk of the phrenic nerve immediately before it penetrates the pleural cavity. De-afferentation was accomplished in the following manner: the spinal ganglia of C5-C7 spinal roots were excised after the laminae of the corresponding segments had been removed. Initially a more extensive laminectomy was made and the ganglia of C3-C8 were crushed and the dorsal roots transected. However, since this procedure proved less effective, more dangerous and was followed by more undesirable side effects, it was abandoned in favor of a radical excision of the critical ganglia. Hemorrhages from the epidural venous plexuses were minimized by the very careful peeling and scraping of the osseous laminae and ligaments. WisconsinHistology. The operated animals were sacrificed 22 to 45 days after operation, and segments of the phrenic nerve from both the intact and the de-afferented sides were removed a few cm above the diaphragm. The nerve segments were splinted on cards, fixed in 1% osmium tetroxide, embedded in paraffin, sectioned at 5 ~1 and mounted according to the method described by Lloyd and Chang ('48). The analysis of fiber diameters was carried out in the following manner: suitable cross sections of both nerves were photographed and enlarged for measurement of fiber diameters. Enlargements of 1,000 X proved most satisfactory. Measurements of fibers were made directly on the photographs using a hand magnifying glass and a rule with 0.5 mm-graduation. A tracing was made of the enlargement, and as each fiber was measured, its diameter was recorded in the proper place on the tracing. When the data were tabulated, each entry on the tracing was simply checked off. This method made it very easy to recheck a particular measurement or area, and virtually eliminated any possibility of counting a fiber twice or missing it entirely. All measurements were made by the same person, and were constantly monitored and checked on the original slide with microscope and ocular micrometer. In general, t...

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