1872.] 



MR. A. H. GARROD ON THE GIZZARD OF BIRDS. 



527 



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 move- 

 ment must have been impossible in them. 



Fig. 1. 





Fig. 2. 



Fig. 1. Horizontal section of the gizzard in a state of relaxation. 

 2. The same in a fully contracted state. 



The following explanation of the action of the gizzard as a simple 

 crushing-organ seems to me much more in accordance with the 

 known principles of animal mechanics. 



As is well known, muscular tissue, when it contracts, does not 

 alter in volume, but gains in breadth what it loses in length 

 during its action. Consequently when a large mass of short mus- 

 cular fibres contracts it must alter its shape considerably, increasing 

 greatly in breadth. 



This fact being borne in mind, the action of the gizzard is easily 

 explained. 



The two enormous lateral muscles, with their fibres tending for- 

 wards and backwards, when relaxed, have a large cavity between 



