122 
MR. SIBSON ON THE BLOW-HOLE OF THE PORPOISE. 
No. 2. Figs. 1.2 (figs. 7-8. Archives of the Royal Society). The anterior inferior 
sac. Muscular. 
Nos. 3.3. Figs. 1.2 (fig. 8. Archives of the Royal Society). The two posterior saes. 
Membranous. The sacs communicate with the nasal canal. 
No. 4. Fig. 2. Semicartilaginous mass in front of the blow-hole. 
No. 5. Fig. 2. Muscle, arising from the frontal bone, inserted into the semicartila- 
ginous mass (4) in front of the blow-hole ; with its fellow it draws that 
mass backwards to close the blow-hole. 
No. 6. Fig. 2. Muscle that, with its fellow, opens the blow-hole by drawing forwards 
the semieartilaginous mass in front of the blow-hole. 
No. 7- Fig. 2. Muscle that dilates the outlet by drawing backwards its posterior 
wall. 
No. 8. Fig. 2. Muscle that compresses and empties the anterior superior sac (1). 
No. 8 a. Fig. 2. Deeper muscle that envelopes, compresses and empties the ante- 
rior superior sac, and dilates the outlet of the blow-hole by drawing for- 
wards and sideways its anterior and lateral walls. 
No. 9. Fig. 2. Fan-shaped muscle that dilates the blow-hole by drawing backwards 
and outwards its posterior and lateral walls. 
No. 10. Fig. 2. Muscle that, with its fellow, arches over, compresses and empties 
the deep anterior sac (2). 
Nos. 11.12.13.14.15 (figs. 4.5.6. Archives of the Royal Soeiety). The dilators of 
the nasal canal. Fan-shaped rnuseles, that arise like No. 9. from the cir- 
cumference of the fronto-faeial ridge, and are inserted into and expand 
various portions of the walls of the nasal canal. 
Nos. 16 . 17 . 18 . 19 (figs. 6 . 7 . 8 . Archives of the Royal Society). Compressors of the 
posterior sacs (3). 
No. 20. Fig. 1 (fig. 8. Archives of the Royal Soeiety). Musele that divides into 
.two tendons (20 and 20 a.) and constricts the opening of the posterior 
sacs (3). 
No. 21. Fig. 1. Cartilage in front of the opening of the posterior lateral sae (3), into 
which is inserted 
No. 21 h (fig. 8. Archives of the Royal Society). A deep muscle that surrounds and 
constricts the opening of the posterior sac (3). 
Nos. 22.23. Fig. 1. Are sections of the tendons of the two following double-headed 
muscles 22 a. 22 h. 23 a. 23 h. 
Nos. 22 a. 23 a (fig. 7- Archives of the Royal Society). Are the two heads that with 
their fellows of the opposite side act from behind forwards to tighten the 
tendons 22.23, and so close the nasal canal below the openings of the ante- 
rior superior sacs (1), and, I conceive, constrict the openings of those sacs. 
Nos. 226.236 (fig. 7- Archives of the Royal Society). Are the two heads that act 
