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Dissection of the thoracic inlet was performed on 22 cadavers to determine the relationship of the phrenic nerve to the internal mammary artery as it passes from lateral to medial behind the first rib. On the left the nerve was found to cross superior to the artery and then medial to it in 14 of 22 specimens; on the right this was found in ten of 22 specimens. In all other specimens, it crossed inferior to the internal mammary artery. These findings demonstrate that there is no constant relationship between these structures, and emphasize the need for caution when dissecting the internal mammary artery at or above the level of the first rib.

The ligamentous attachments of the lumbar spinous processes were studied by dissection and histological examination in seven dogs, three cats and six baboons. Unlike man, the lumbar spines of these animals do not possess a supraspinous ligament and there is no decussation of the erector spinae tendons in the lower lumbar region. In the baboon the lumbar interspinous ligaments are thick bilateral structures composed of elastic fibres running in a direct craniocaudal direction (Fig. 5). In the dog and cat the interspinous ligaments are rudimentary, with very thin ventral and middle parts, but with a somewhat better developed dorsal part, most of whose fibres run obliquely dorsocranially as in man, but with some elastic fibres in addition which run directly craniocaudally as in the baboon. The baboon is clearly intermediate between man on the one hand and the dog and cat on the other in these respects, and this would appear to reflect the intermediate requirements of the baboon in relation to the control of flexion of the lumbar spine during locomotion.