1078 
DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
is found on eacli side of the neck, at the back of the jugular vein (see Plate 87 , fig. 
29) ; in the Reptile, lying on the vessels near the base of the heart, as in the Alligator 
or the Snake; or situated near the carotid artery, as in the Tortoise ; in Amphibia, at 
the back of the angle of the jaw; in the Ray-fish, lying behind the spout cavity, and 
between the gills and the muscles of the back (see Plate 84, fig. 5, Th.). 
Its appearance in Mammals is always the same— i.e,, a grey white mass divided 
up into lobules and follicles. In the Bird, it appears as an elongated mass, which to 
the naked eye contains a central band, on which the follicles are arranged; in the 
Reptile, as a small oval pale yellow mass, a little whiter than the fat of the animal; in 
Amphibia, as a white oval body; in the Ray-fish, as a grey-white triangular mass, 
very soft in texture, but in the adult animal showing the individual follicles to the 
naked eye. 
In the following pages, the thymus, as it is found in the Mammal, will be described ; 
and afterwards the various differences and points of agreement between the thymus 
of the Mammal and that of other Vertebrates will be pointed out. 
The thymus is surrounded by a connective-tissue envelope ; this covering sends in 
membranous projections, which penetrate between the separate lobes, and to a very 
great extent between the lobules ; and penetrating into the lobules, mark out the 
division into follicles (see Plate 83, fig. 1). In the connective-tissue run the blood 
and lymphatic vessels, and a few nerves. 
If we attempt with the scalpel and forceps to divide up the thymus, and select the 
thymus of the Calf, as being from its great: size the easiest to deal with, it will be 
found that we can easily separate the gland into lobes, and these again into smaller 
lobes. In the lobes, the lobules are marked out to the naked eye, especially in 
portions of the gland which have been allowed to lie for some hours in 2 per cent, 
bichromate of potash. They are generally four-sided, and somewhat pyramidal in 
form (see Plate 87, fig. 24). The attempt to separate the lobes into lobules is not 
however always successful, as though some lobules are unconnected, in most cases they 
are attached to one another or to a central string which runs up the cervical 
portion of the thymus, as was known and depicted by various authors including 
Sir A. Cooper, (25). This central string consists of connective-tissue and of blood 
vessels. The lobules are seen by the naked eye to be composed of follicles, but it is 
not possible to separate the individual follicles, as they are almost always attached to 
one another (see Plate 83, figs. 1, 2; and Plate 84, fig. 3). 
Anatomy oe the Follicle as a whole. 
The follicles are, in the fully-developed thymus, chiefly of a polygonal form (see 
Plate 83, fig. l). As, however, they are usually attached to one another, their forms 
are somewhat various. In some cases, compound branched follicles are met with (see 
Plate 83, figs. 1,2; and Plate 84, fig. 3). 
