The Smooth Facial Muscles of Anura and Salainandrina. 333 



in both, however, the epithelium is thinner than on the general surface 

 of the body. 



The structure which is characteristic of the cutis of the external 

 skin is also to be observed in the outer layer of the upper lip, but 

 especially in the dorsal portion of this layer, where the usual hori- 

 zontal and vertical bundles are well developed. Toward the margin 

 of the lip, the horizontal layer thins out and the structure gradually 

 becomes reduced to a simple network of connective tissue surrounding 

 the sack-like glands. In the inner layer of the lip the cutis consists 

 wholly of a dense cushion of connective tissue without definite 

 arrangement. Between the two connective tissue strata just described 

 we find a thick band of smooth muscular tissue, whose upper margin 

 lies external to the lower edge of the dentary processes of the bones 

 of the jaw, while its lower border approaches close to the ventral 

 margin of the lip. This muscle, which I name muse ul us la- 

 bialis superior (Figs. 9 and 10, PI. XVII and Figs. 11, 12 and 15, 

 PI. XVIII), extends backward on each side to the posterior end of 

 the maxillare, where it terminates at the syndesmosis between the 

 maxillare and the quadrato-jugale. Posteriorly the muscle decreases 

 in size, corresponding to the decrease of the lip itself. The labialis 

 superior is composed of parallel, freely anastomosing bundles of 

 smooth muscular tissue, whose general direction conforms with that 

 of the lip. Excepting at the caudal extremities of the muscle, these 

 bundles seem to be wholly independent of the skeletal parts. The 

 component bundles of the muscle are inclosed in a matrix of con- 

 nective tissue which also immediately envelopes the entire muscle 

 and conveys the blood vessels and nerves. 



The labialis superior has been observed in Rana, Hyla, Alytes, 

 Bombinator and Bufo, in all of which it is a well developed muscle. 

 In Bufo, with its toothless upper jaw, the form of the muscle is 

 somewhat different from that described above, the vertical diameter 

 of the muscle being less than in Rana. 



c. The Mechanism in Operation. 



The movability of the intermaxillaria and the partes mentales 

 of the dentalia, which was involved in their utilization for opening 

 and closing the external nares in Rana, left the masticatory muscles 

 incompetent to maintain a close contact of the jaws during in- 

 spiration and necessitated the introduction of various structural changes 



23* 



