The =deep fascia= also forms two layers. The =superficial layer= is attached to the wing of the atlas and the lower edge of the trachelo-mastoideus and scalenus. Passing downward, it incloses the trachea, and, together with the deep layer, furnishes sheaths for the vagus and sympathetic nerves and the carotid artery. Passing upward it detaches septa between the extensor muscles of the spine. Anteriorly it covers the thyroid gland, the guttural pouch, the adjacent vessels and nerves and the larynx, and is attached to the mastoid process of the temporal bone and the thyroid cornu of the hyoid bone. Posteriorly, it is attached to the first rib and the cariniform cartilage of the sternum. The =deep layer= (prevertebral fascia) covers the ventral surface of the longus colli, and incloses the trachea and œsophagus. Anteriorly, it forms, with the corresponding layer of the opposite side, a septum between the guttural pouches; posteriorly, it becomes continuous with the endothoracic fascia. A =fascia propria= forms a tubular sheath around the trachea, inclosing also the recurrent nerves. VENTRAL MUSCLES This group consists of eleven pairs of muscles which lie almost entirely ventral to the vertebræ. 1. =Panniculus carnosus.=—The =cervical panniculus= (platysma myoides of man) has a fleshy origin on the cariniform cartilage (manubrium) of the sternum. it passes forward, outward, and upward, crossing over the sterno-cephalicus and jugular furrow obliquely. On reaching the surface of the mastoido-humeralis it adheres closely to this muscle, and soon thins out, to be continued over the splenius and trapezius by an aponeurosis which is difficult to remove from the latter muscle. Scattered bundles may be traced on the ventral surface of the neck to the facial portion. The right and left muscles meet at a ventral median raphé in pennate fashion. [Illustration: FIG. 177.—ANTERO-LATERAL VIEW OF MUSCLES AND SKELETON OF HORSE. _a_, Trapezius; _c_, mastoido-humeralis; _d_, sterno-cephalicus; _f_, long head of triceps; _f′_, external head of triceps; _g_, anterior superficial pectoral muscle; _g′_, posterior superficial pectoral; _h′_, anterior deep pectoral; _v_, cervical panniculus; _z_, supraspinatus; _29_, omo-hyoideus; _30_, sterno-thyro-hyoideus; _31_, jugular vein; _32_, cephalic vein; _1_, scapula; _1′_, cartilage of scapula; _2_, spine of scapula; _4_, shaft of humerus; _4′_, external epicondyle; _5_, external tuberosity of humerus; _6_, deltoid tuberosity; _14_, ventral border (“keel”) of sternum; _14′_, cariniform cartilage; l.R., first rib. (After Ellenberger-Baum, Anat. für Künstler.) ] 2. =Mastoido-humeralis.=—This is described on p. 252.