xi PRACTICAL DIRECTIONS 553 



5. The flaps of the tricuspid and bicuspid valves and their 

 tendinous cords and papillary muscles (Fig. 139). 



6. The apertures of the aorta and pulmonary artery into 

 the left and right ventricle respectively. 



Sketch. 



F. Dissection of the head. 



I. Carefully dissect the skin away from one side of the 

 head ; either side will do, but if you are using the head from 

 which you have already removed the brain, choose the side 

 on which the auditory region is least damaged : notice the 

 lleibomian glands (p. 535). Cut away, with bone-forceps, 

 the supraorbital process of the frontal, being careful not to 

 injure any of the contents of the orbit. Note 



1. a, A large mass of muscle (masseter] covering the 

 posterior half of the mandible, on which branches of the 

 facial nerce will be seen ; b, the thin irregular parotid gland, 

 at the base of the pinna, and the large infraorbital gland 

 lying mainly within the orbit below (p. 512). 



2. The four recti and the two oblique muscles of the eye. 

 Note the course of the superior oblique through its tendinous 

 loop (p. 534). 



3. The lacrymal and Harder ian glands, situated in the 

 postero-superior and antero-inferior regions of the orbit 

 respectively. 



II. Remove the eyeball by cutting through the muscles 

 and optic nerve, noting the retractor bulbi muscle around the 

 latter. (For the dissection of the eye, see p. 555). Cut off 

 the pinna, and clear away the muscles, &c., covering the 

 tympanic bone : remove the entire tympanic and periotic 

 bones (p. 492) in one piece, lay open the external auditory 

 passage, and very carefully and gradually cut away the 

 outer wall of both tubular and bulbous portions of the 

 tympanic with the bone-forceps. Note : 



1. The tympanic membrane and the handle of the malleus ; 

 and after cutting away the former so as not to injure the 

 latter, and removing rather more of the bulla, make out 

 (Fig. 143) 



2. The tympanic cavity; the malleus, incus, and stapes; 

 the tensor tympani and stapedius muscles ; and the aperture 

 of the Eustachian tube. Remove the auditory ossicles, 

 noting as you do so the fenestra ovalis and fenestra rotunda, 

 and examine them under the low power of the microscope : 

 by cutting through the periotic with the bone-forceps, the 

 position of the membranous labyrinth, including the cochlea, 

 may be made out. 



PRACT. ZOOL. M M 



