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 open ; 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 



