Dissection of Common Rat, 181 



the thumb, and unguiculate. The Hind-feet are pentadactyle. 

 The Incisor teeth are two in number in each jaw, large, in- 

 curved, and separated from the Molars by an interval or dias- 

 tema, the result of the withdrawal of the Canines : their depth 

 is greater than their width, and their sides are somewhat 

 flattened. 



The creature before us bears the following external charac- 

 teristics of a YERTEBRATE : its body seems bilaterally 

 symmetrical, and presents no appearance of segmentation ; 

 it has not more than two pairs of limbs ; its jaws appear to 

 be modifications of the walls of the head rather than modifi- 

 cations of limbs ; the organs of smell, sight, and hearing, 

 are limited to the head, and consist of single bilaterally sym- 

 metrical pairs ; lastly, it is possessed of an anus, no Verte- 

 brate being aproctous. 



It bears the following characteristics of a MAMMAL, viz.: 

 the covering of hair ; the vibrissae on the snout ; the under- 

 jaw alone moveable ; the external ear ; the cartilaginous pro- 

 longation of the bony parts of the nasal cavity, forming a 

 nose, or muzzle : and the intromittent organ of generation. 



It would be placed with the Rodkntia, as being possessed 

 of two scalpriform Incisors in each jaw, and having these 

 separated by a diastema from the molars, i.e. lacking Canine 

 teeth : and from its scaly thinly- haired tail, it would be 

 grouped with the Muridce. 



Injection. The animal is best injected from the jugular 

 vein, which may be readily found by first making a straight 

 incision through the skin, about an inch long, from the end of 

 the sternum to the symphysis of the jaw, and reflecting the 

 the skin on one side ; and then with the scalpel making 

 a slight incision through the muscle underlying the skin ; 

 when the vein will be found holding an antero-posterior 

 course across the clavide about half way between the middle 

 of the chest and the anterior border of the shoulder. A 



