Page:EB1911 - Volume 21.djvu/381

Rh The Bursa pharyngeal was at one time looked upon as the place whence the pituitary body had been derived from the roof of the pharynx, but this is now disproved and its meaning is unknown. The tonsil is formed in the second bronchial cleft or rather pouch, for the clefts are largely incomplete in man, about the fourth month; its lymphoid tissue, as well as that elsewhere in the pharynx, is formed from lymphocytes in the subjacent mesenchyme (see ), though whether these wander in from the blood or are derived from original mesenchyme cells is still doubtful. The development of the ventral part of the pharynx is dealt with in the articles and.

For literature see Quain's Elements of Anatomy, vol i. (London, 1908), and J. P. McMurrich, Development of the Human Body (London, 1906).

Comparative Anatomy.—In the lower, water-breathing, vertebrates the pharynx is the part in which respiration occurs. The water passes in through the mouth and out through the gill slits where it comes in contact with the gills or branchiae.

The lowest subphylum of the phylum Chordata, to which the term Adelochorda is sometimes applied, contains a worm-like creature Balanoglossus, in which numerous rows of gill slits open from the pharynx, though Cephalodiscus, another member of the same subphylum, has only one pair of these.

In the subphylum Urochorda, to which the Ascidians or sea squirts belong, there are many rows of gill slits, as there are also in the Acrania, of which Amphioxus, the lancelet, is the type. In all these lower forms there are no true gills, as the blood-vessels lining the large number of slits provide a sufficient area for the exchange of gases.

In the Cyclostomata a reduction of the number of gill slits takes place, and an increased area for respiration is provided by the gill pouches lined by pleated folds of entodermal mucous membrane;

these form the simplest type of true internal gills. In the larval lamprey (Ammocoetes) there are eight gill slits opening from the pharynx, but in the adult (Petromyzon) they are reduced to seven, and a septum grows forward separating the ventral or bronchial part of the pharynx from the dorsal or digestive part. Both these tubes, however, communicate near the mouth.

In fishes there are usually five pairs of gill slits, though a rudimentary one in front of these is often present and is called the spiracle. Occasionally, as in Hexanchus and Heptanchus, there may be six or seven slits, and the evidence of comparative anatomy is that fishes formerly had a larger number of gill slits than at present.

In the Teleostomi, which include the bony fishes, there is an external gill cover or operculum.

In the Dipnoi or mud fish the work of the gills is shared by that of the lungs, and in the African form, Protopterus, external gills, developed from the ectodermal parts of the gill slits, first appear. In the tailed Amphibians (Urodela) the first and fifth gill clefts are never perforated and are therefore in the same condition as all the gill clefts of the human embryo, while in the gilled salamanders (Necturus and Proteus) only two gill clefts remain patent. The gills in all the Amphibia are external and of ectodermal origin, but in the Anura (frogs and toads) these are succeeded before the metamorphosis from the tadpole stage by internal gills, which, unlike those of fish, are said to be derived from the ectoderm.

In the embryos of the Sauropsida (reptiles and birds) five gill clefts are evident, though the posterior two are seldom at any time perforated, while in the Mammalia the rudiments of the fifth cleft are no longer found in the embryo, and in man, at all events, none of them are normally perforated except that part of the first which forms the Eustachian tube. It will thus be seen that in the process of phylogeny there is a gradual suppression of the gill clefts beginning at the more posterior ones.

The soft palate is first found in crocodiles as a membranous structure, and it becomes muscular in mammals. The bursa pharyngea and pharyngeal tonsil are found in several of the lower mammals. In the sheep the latter is particularly large.

For literature and further details, see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907); also Parker and Haskell's Zoology (London, 1897).

 PHEASANT (Mid. Eng. fesaunt and fesaun; Ger. fasan and anciently fasant; Fr. faisan—all from the Lat. phasianus or phasiana, sc. avis), the bird brought from the banks of the river Phasis, now the Rioni, in Colchis, where it is still abundant, and introduced, according to legend, by the Argonauts into Europe. Judging from the recognition of the remains of several species referred to the genus Phasianus both in Greece and in France, it seems not impossible that the ordinary pheasant, the P. colchicus of ornithologists, may have been indigenous to this quarter of the globe. If it was introduced into England, it must almost certainly have been brought by the Romans; for, setting aside several earlier records of doubtful authority, Stubbs has shown that by the regulations of King Harold in 1059 unus phasianus is prescribed as the