698 MAMMALIA. 



Mammalia have a seventh, called the choanoid or funnel-shaped 

 muscle. This likewise arises from the borders of the optic fora- 

 men, and, gradually expanding, forms a hollow cone interposed be- 

 tween the recti muscles and the optic nerve ; the base of the cone 

 being attached to the sclerotic, behind the insertion of the recti. 

 Frequently, indeed, this choanoid, or suspensory muscle, is di- 

 vided into four portions, in which case the animals so provided 

 would seem to have eight recti muscles. 



(812.) The eye-lids of Mammalia resemble the human in every 

 respect, excepting that in the lower orders a remnant of the nicti- 

 tating membrane is still met with ; but it is of small dimensions, 

 and unprovided with muscles. 



(813.) The lacrymal apparatus exists in all quadrupeds, and 

 the lacrymal gland occupies the same situation as in Man; the tears 

 being poured on to the conjunctiva near the external canthus of 

 the eye-lids. The lacrymal ducts, likewise, whereby the tears are 

 conveyed into the nose, so nearly resemble the human as to re- 

 quire no particular description. The carunculte, lacrymales are 

 also met with at the inner canthus of the eye-lids. In some 

 quadrupeds, indeed, an additional gland exists, called the glandula 

 Harderi : this is situated behind the internal angle of the eye, 

 and secretes a lubricating fluid, that is discharged beneath the 

 rudiment of the third or nictitating eye-lid. 



(814.) In Whales, as might be expected from their aquatic 

 Jhabits, no vestige of a lacrymal apparatus is to be seen. 



(815.) Behind the optic lobes of the encephalon the nervous 

 centres, from whence the other cerebral nerves take their origin, 

 are so intimately blended together, that the anatomist is no longer 

 able to distinguish them from each other. They form, in fact, 

 the " medulla oblongata" and are the commencement of that 

 long series of sentient and of motor ganglia that forms the spinal 

 cord. 



All the nerves derived from the medulla oblongata, and from 

 the spinal cord, are throughout the Mammiferous class exactly 

 comparable to those met with in our own species, and therefore 

 will require but brief notice. 



(816.) The third, fourth, and sixth pairs are destined to the 

 muscles of the eye, and their distribution is the same as in Man. 



(817.) The fifth pair, or trigeminal nerves, consist of both 

 motor and sentient fasciculi, both of which are distributed to the 

 different parts of the face exactly as in the human subject ; allow- 



