448 Royal Society. 
in the rows of epithelial cells surrounding the central axis of the 
lobule, and that the fluid there secreted is transmitted to the cells 
forming the margin of the lobule, where it is further elaborated, 
and, by the bursting of these cells, is conveyed into the cavity of the 
surrounding duct. A few diagrams are annexed, illustrative of the 
descriptions of microscopic structure given in the paper. 
Feb. 19.—“ On the Mechanism of Respiration.” By Francis Sib- 
son. Communicated by Thomas Bell, Esq., F.R.S. 
This paper is almost entirely occupied with anatomical details, 
collected from an extensive series of dissections of the muscles and 
bones concerned in the act of respiration in man and the lower ani- 
mals, for the purpose of elucidating the mechanism of their action 
both in inspiration and in expiration ; accompanied by a great num- 
ber of illustrative diagrams and drawings. ‘The author commences 
with the serpent tribes, which present the simplest form of ribs, 
being attached only at their vertebral ends, while their anterior ends 
are free. When these ribs are brought forwards by the action of 
the levatores costarum and external intercostal muscles, the chest is 
expanded ; and when drawn backwards by the long depressors, in- 
ternal intercostals and transversales, expiration is effected. In birds 
there are added to the former apparatus a sternum, and a series of 
sternal ribs, the respiratory movements of which are performed in 
directions the reverse of those of the vertebral ribs. During inspi- 
ration, the angles between the vertebral and sternal ribs become 
more vpen; the sternum moves forwards, and the spinal column 
slightly backwards, by the combined action of the scaleni and 
sterno-costal muscles on the first vertebral and first sternal ribs re- 
spectively ; of the levatores costarum and external intercostal on all 
the lower vertebral ribs, and of the sternal intercostals on all the 
lower sternal ribs. On expiration these movements are reversed by 
the action of the internal intercostals, the external and internal 
oblique, recti, transversales and other muscles. The mechanism in 
the Mammalia is further assisted and modified by the addition of a 
large and powerful diaphragm. ‘The thoracic ribs are articulated 
with the sternum by the medium of cartilages corresponding to the 
sternal ribs of birds: those ribs which are connected with the infe- 
rior curve of the dorsal arch have floating cartilages, and may be 
considered as a diaphragmatic set of ribs. When raised, the former 
approach each other, and the latter recede from each other ante- 
riorly. Intermediate to these are the longer ribs connected with the 
dorsal arch, having their cartilages united, and articulated with the 
lower end of the sternum. The scaleni muscles invariably act during 
the whole time of inspiration. ‘The external intercostals between 
the thoracic ribs are also throughout inspiratory ; but those portions 
which are situated between their cartilages are expiratory; and 
those between the diaphragmatic ribs are inspiratory behind, expi- 
ratory to the side, and in front, and inspiratory between their carti- 
lages. Between the intermediate ribs, they are for the most part 
slightly inspiratory between the ribs, and expiratory in front, be- 
tween the cartilages. The external intercostals of the thoracic ribs 
are expiratory behind, and inspiratory in front, if the ribs approach 
