106 THE MECHANISM OF THE GIZZARD IN BIRDS. 
the corresponding cavities already mentioned; and the cesophagus 
enters the former at its inner angle, close to its junction with the 
right lateral mass ; the duodenum being behind. 
The lateral masses, with their tendinous coverings, are the mus- 
cular portions; and the cavity between them is just behind the 
central tendon. 
The epithelial lining of the whole organ is very dense, and is con- 
tinuous through the different cavities, terminating abruptly at the 
entrances to the cesophagus and the duodenum. Very shortly after 
the death of the bird it can be stripped off entire.* It is particularly 
dense where it covers the two lateral muscles, and generally forms a 
callous oval pad over each, which has to receive most of the force of 
the muscular walls as they act on the stones and food. 
The central tendons, one in front and the other behind, are very 
strong; and so are the fibres which radiate outwards from them; 
they are almost entirely connected with the lateral muscles. 
The lateral muscular masses have their fibres all tending forwards 
_ and backwards, each being inserted into both the front and back 
Page 527. 
tendinous expansion, the central being nearly straight and the lateral 
ones being curved slightly outwards in the middle of their course. 
The superior and inferior sacs are surrounded by muscular bands 
which bow over from front to back, being inserted into those parts of 
the margin of the central tendon to which they are opposite. By 
their contraction they reduce the size of the sacs and force any thing 
they contain between the lateral muscles, a considerable fold of the 
gizzard-lining, which acts as a kind of valve, preventing any stones 
entering the duodenum. 
The action of the lateral muscles can be best understood by observ- 
ing a horizontal section made through the middle of the gizzard. 
The section is fusiform and exhibits a central oblong cavity, short 
from side to side, bounded before and behind by the central tendons, 
and laterally by the triangular muscular masses. 
The accompanying figure and the above description show that in 
the gizzard there is no mechanism which could in any way produce 
any lateral movement of the one mass of muscles on the other; and it 
is difficult to conceive any epithelium, however horny and dense, that 
could resist the tearing-strain which would necessarily be associated 
with such movement, in addition to which several gizzards that have 
passed through my hands have been so loaded with fat or adherent to 
the abdominal walls, that any lateral movement must have been 
impossible in them. 
* This coat is considered by recent German authorities to be a secretion from 
the deep glands, not an epithelium. 
