The erectores spinae muscles are described by topographical anatomists as extensors of the trunk. Borelli (1710), Duchenne (1867) and Beevor (1904) asserted that they contract when the trunk is flexed from the upright position and so act as antagonists to gravity. Weddell, Feinstein & Pattle (1944) stated that the lumbar and thoracic sacro-spinalis muscles are easily relaxed with ' satisfactory positioning', but gave no details of postures in which this occurs. Akerblom (1948) reported briefly that the electromyogram of the lumbar sacro-spinalis muscle, in eight out of twelve subjects sitting in the sunken position (i.e. with full flexion of the trunk), showed practically no difference from the resting electromyogram. Allen (1948) stated that the erector spinae is quiescent when full trunk flexion has been reached from the orthograde position. Kelton & Wright (1949) found in two subjects that the erectores spinae muscles were electrically silent for long periods of time in the 'easy standing position '. Floyd & Silver (1950 described the action of the erectores spinae during 'straining' and in flexion and extension of the trunk.The present paper describes the functions of the erectores spinae muscles in certain postures and movements and during weight-lifting, as studied in 150 human subjects by electromyographic, photographic and radiographic methods. A preliminary account of this work has been given in brief communications to the Anatomical and Physiological Societies during 1949-52. METHODSIn most experiments surface electrodes were placed directly over the muscles in the lumbar region. Concentric needle electrodes were also used in order to record the activity of the deeper parts of the muscles. The action potentials were amplified and recorded with either a 4-or 6-channel Ediswan electroencephalograph.

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It seems to be generally accepted that experimenting in ovo on the chick during the early stages of development (up to about 48 hours) is fraught with the greatest difficulty. After about this time no serious technical problems arise and a high proportion of successful results can be expected. It is natural to ask why there should be this change-over from extreme difficulty to reasonable simplicity. New (1955) attributed to this ‘inaccessibility of the chick embryo in the egg’ the invention of his own and many other in vitro methods during the last 30 years. There is no doubt that, when short-term experiments only are required, in vitro methods will probably always be preferred. But all in vitro methods suffer from the disadvantage that the embryo cannot be expected to survive for more than 48 hours or so after explantation.

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The present communication concerns the influence in the chick (Gallus domesticus) of the growing eyes on various structures which form the wall of the orbits, or which lie in their vicinity. Features of the development of the trabeculae cranii, the fronto-nasal process, the nasal capsules, the maxillary ‘process’, the otic capsule, and the cephalic flexure will be described as well as certain growth relationships of the upper and lower jaws. The methods employed include (1) removal of one or both primary optic vesicles, (2) the grafting of an additional primary vesicle into the head mesenchyme so that two eyes develop in one orbit, and (3) the isolation in the coelom of various head components. The importance of the trabeculae to our present study stems from the fact that they contribute to the boundaries of the orbits on their medial sides.