Part 159

MEDIASTI´NUM, n. One of the thoracic partitions.

MEDUL´LA, n. Marrow. Medulla spinalis, spinal cord. Medulla oblongata, tract of nerve matter between cerebellum and spinal cord proper.

MEL´ANISM, n. State of coloration resulting from excess of black or dark pigment; a frequent condition of hawks.

MELANIS´TIC, MELANOT´IC, a. Affected with melanism.

MEM´BRANA PUTAM´INIS, n. The lining membrane of the egg-shell, formed of dense modified albumen.

MEM´BRANE, n. Thin soft sheet of various structure covering a part or organ. Cerebral or spinal membrane. See MENINGES. Nictitating membrane, the third or inner eyelid of birds, which sweeps across the ball. Soft skinny covering of the bill of many birds is said to be membranous. Webbing of the toes is the interdigital membrane. Loma is a fringed membrane.

MEM´BRUM, n.; pl. membra. Any limb, or other peripheral part, as a bill, as distinguished from body proper or truncus.

MENIN´GES, n. pl. Envelopes of the brain or spinal cord; especially the dura mater and pia mater. (The singular, meninx, is scarcely used.)

MEN´TAL, a. Pertaining to the chin.

MEN´TUM, n. Chin; soft parts between the branches of the lower jaw.

MESENCEPH´ALON, n. A certain brain-tract, the second from behind.

MESENTER´IC, a. Pertaining to the mesentery.

MES´ENTERY, n. A fold of the peritoneum binding the intestines in place.

MESERA´IC, a. Same as MESENTERIC (which see).

ME´SIAL, a. In the middle; same as median.

MESO- (in composition). Middle; median.

MESOMET´RIUM, n. Partially muscular peritoneal fold supporting the oviduct.

MESORHI´NIUM, n. Portion of bill between the nostrils. (Little used.)

MESOSTER´NUM, n. Middle segment of the breast-bone.

METACAR´PAL, a. or n. Pertaining to the metacarpus; particularly the metacarpal bone.

METACAR´PUS, n. Hand, exclusive of the fingers; segment of the wing between the carpus and the digits.

METAGNATH´OUS, a. Cross-billed; having the points of the mandibles passing each other on the right and left.

METATAR´SAL, a. Pertaining to the metatarsus; particularly the metatarsal bone.

METATAR´SUS, n. Foot, exclusive of the toes; segment of the leg between the tarsus and the digits, commonly called the shank, and in descriptive ornithology usually known as the tarsus. The metatarsus, however, has a distal tarsal ossicle confluent with it, so that it is actually tarso-metatarsus. In birds, the metatarsus proper (exclusive of the confluent tarsal bone) consists of three parallel metatarsal bones, more or less completely confluent, and of the associated accessory metatarsal bone which bears the hallux. Compare TARSUS.

METATAR´SUS HAL´LUCIS, n. The accessory metatarsal bone, as just said.

MID´DLE TOE, n. The third toe in order of reckoning, with few exceptions four-jointed. When the fourth toe is reversed, as in all zygodactyle birds except trogons, it becomes the outer anterior toe; in trogons, and a few birds in which the true inner or second toe is wanting, it becomes the inner anterior toe. It is never versatile. It rarely has only three joints, like the second toe. (128.)

MID´DLE WING-COV´ERTS, or ME´DIAN COV´ERTS. The series of upper coverts of the secondary set, situate in one or more rows between the greater and lesser coverts. They are usually recognized by their overlapping each other in the reverse direction (i.e. inner border of one overlapping outer border of the next one) from the others, whence they are sometimes called tectrices perversæ. (94.)

MIGRA´TION, n. Periodical (but sometimes irregular) journeyings, or change of abode, of birds at certain seasons, to secure food, climate, or other physical conditions of environment best suited to their wants. Migration is generally meridional (north-south), and believed by some to be mainly accomplished along a magnetic meridian; but it is often quite otherwise, influenced by topography, etc., or altogether capricious. In the Northern Hemisphere, the vernal migration is northward, the autumnal in the opposite direction.