When the neck is extended in surgical position, as seen in Plates 3 and 4, its general outline assumes a quadrilateral shape, approaching to a square. The sides of this square are formed anteriorly by the line ranging from the mental symphysis to the top of the sternum, and posteriorly by a line drawn between the occiput and shoulder. The superior side of this cervical square is drawn by the horizontal ramus of the lower maxilla, and the inferior side by the horizontal line of the clavicle. This square space, R 16, 8, 6, Plate 4, is halved by a diagonal line, drawn by the sterno-cleido-mastoid muscle B, which cuts the square into two triangles. In the anterior triangle, F 16, 6, Plate 4, is located the superficial common carotid artery, C, and its branches, D, with accompanying nerves. In the posterior triangle, 9, 8, 6, Plate 4, is placed the superficial subclavian artery, A, its branches, L M, and the brachial plexus of nerves, I. Both these triangles and their contents are completely sheathed by that thin scarf-like muscle, named platysma myoides, A A, Plate 3, the fibres of which traverse the neck slantingly in a line, O A, of diagonal direction opposite to and secant of that of the sterno-mastoid muscle. When the skin and subcutaneous adipose membrane are removed by careful dissection from the cervical region, certain structures are exposed, which, even in the undissected neck, projected on the superficies, and are the unerring guides to the localities of the blood-vessels and nerves, &c. In Plate 4, the top of the sternum, 6; the clavicle, 7; the "Pomum Adami," 1; the lower maxilla at V; the hyoid bone, Z; the sterno-cleido-mastoid muscle, B; and the clavicular portion of the trapezius muscle, 8; will readily be felt or otherwise recognised through the skin, &c. When these several points are well considered in their relation to one another, they will correctly determine the relative locality of those structures--the blood-vessels, nerves, &c., which mainly concern the surgical operation.