Showing nervous, digestive, circulatory, and parts of respiratory and renal systems.
The brain has been exposed in situ by the removal of the roof of the cranium; the integument has been removed from the right side of the front of the cervical region, as have also most of the feathers from the entire body; an opening has been made into the right side of the crop, which has been distended; the larger part of the right half of the body walls has been removed, together with the muscles and the limbs which it supported, and a red injection has been thrown into and filled the venous system.
The surface of the cerebral hemispheres is smooth; the proportion of the encephalic nervous mass to the intraspinal is much greater than in the cold-blooded Vertebrata. The backward projection of the cerebellum is very considerable. The eyes are large. The vertical third eyelid is drawn forward. The nostrils open externally as long slits overhung by a soft, bare, tumid membrane1; the external auditory meatus, which has no concha, has the feathers arranged round it like a circlet of tentacles. The great pectoral muscle, the main depressor of the humerus and the wing, is seen in section along its origin from the lower portion of the keel of the sternum, and from the furculum, the outer and lateral portions of the sternum, from which it also took origin, having been removed. Placed dorsally with reference to this muscle we see the second pectoral, the main elevator of the humerus and the wing, arising from a larger portion both of the keel and of the lateral parts of the sternum than the pectoralis major, and passing internally to the coracoid to enter the pulley-like canal, the foramen triosseuni, formed by the clavicle or furcula, the coracoid, and the scapula.
This muscle is supplied by nerves which pass in front of, whilst the great pectoral is supplied by nerves which pass below the coracoid, the first being homologous with the subclavian, the second with the anterior thoracic nerves of anthropotomy.
Whilst the tendon of the second pectoral or great levator humeri muscle, which is cut short, is seen issuing from its canal on the further side of the glenoid articular surface; on the proximal side, the humerus having been removed, we see the tendon of the biceps, homologous with the 'short head' of anthropotomy. The cut-short triangular end of the pectoralis major is seen to become partially bifid toward its apex; in the perfect condition of the parts the smaller inferiorly-placed division of the muscle gave off two tendons1, one to the long and the other to the short extensor plicae alaris anterioris; the larger division passed over a smooth facet on the humerus and over the coracoid head of the biceps to be inserted upon the inwardly-looking surface of the great triangular tuberosity of the humerus. Dorsally to the apex of the great pectoral we see a thin stratum of muscle in relation internally with the crop and homologous with the deltoid of anthropotomy. This muscle is divided into two strata by delamination.
The superficial layer consists of three parts, of which the first and most internally-placed joins the long extensor of the anterior fold of the alar membrane; the second and mesially-placed joins the short extensor, whilst the third and dorsally-placed portion is inserted into the outer aspect of the humerus from its middle down to a nodule at its lower fourth marking the origin of the long radial extensor of the carpus. The deep layer consists of one short muscle2 innervated by the circumflex, arising from the portions of the coracoid and scapula and of the ligaments in relation with the shoulder-joint, and inserted into the upper surface of the humerus along a line reaching from the apex of the triangular tuberosity receiving the tendon of the great depressor humeri to the facet receiving the tendon of the great long levator. Overlapped by this muscle, which acts as a levator humeri, and wedged between it and the great pectoral depressor, is a second short levator humeri, innervated as is the coracobrachialis, not as is the deltoid, arising from the coracoid and passing down on the outer side of the tendon of the biceps to be inserted under the upper portion of the tendon of the great pectoral.
This muscle therefore should be considered to be a coracobrachialis.
1 This membrane is sometimes called a 'cere,' but it is better to restrict the term to the denser structure similarly placed in and similarly distinctive of the Aetomorphae and Psittacomorphae. Some of the Charadrii-morphae (Plovers), which on account of a peculiarity in the nasal bones (in which they resemble the Pigeons) were placed with them in a separate order, the Charadriiformes, s. Schizorhinae, resemble them also in the conformation of this membrane. See Coues, Key to American Birds, 1872, p. 26; Strickland and Melville, The Dodo and its Kindred, p. 46; Garrod, P. Z. S. 1873, p. 33, 1874, p. 100.
We have thus three muscles - the 'pectoralis secundus,' or long levator; and two shorter muscles, the former of which may be called 'deltoides externus,' and the latter 'coracobrachialis brevis' - entrusted with the work of raising the humerus, but each with a distinct innervation. In some birds, e.g. Anser, the deltoides externus passes into and takes an enlarged origin from the walls of the foramen triosseum, and gains some mechanical advantage by availing itself of its pulley-like outlet.
In relation with the lower portion of the right coracoid may be seen two other coracobrachiales (cut short in this preparation, but shown in situ at v and u, in Plate ii, infra), the pectoralis tertius auctorum, s. coracobrachialis inferior, No. 16; Schoepss; and the coracobrachialis superior. The 'coracobrachial muscular apparatus' has been shown by Professor Wood, Journal of Anatomy and Physiology, i. 1866, p. 55, to be somewhat similarly multifid in many mammals normally and in man occasionally.
1 The alar extensor muscles and many of the other muscles of the wing in Birds will be found well figured and described in a monograph by Schoepss in Meckel's Archiv, 1829. Those of the Pigeon are similarly figured and described by Macgillivray in his History of British Birds, i. pp. 34-42, Plate iii, a work with which I was not acquainted when the first edition of this book was published. Mr. Macgillivray remarks, p. 38, that the small muscular mass called by him retractor plicae, and figured here, Pl. ii. w', infra, had not been met with by him in any other birds except Pigeons.