=Structure= (fig. 122).--Note that the snake is covered with horny _scales_ somewhat as the fish is. How do these scales differ from those of the fish? In snakes the scales are not bony, but are true skin structures. Note the modification of the scales on the head, back, and ventral surface. Those on the dorsal surface often have minute ridges, the _keels_. How do the ventral scales differ from the dorsal ones and others? By a system of muscles these ventral scales are rhythmically moved and as their posterior edges are pushed back against some resisting object the body glides forward. On the head note the pair of _eyes_. Are there eyelids? In front of each eye note an opening. What are these openings? Thrust a bristle into the opening and see where it enters the mouth-cavity through the _internal nares_. Does the snake have external ears? Observe the very long _jaws_ and note that they are loosely hinged. Examine the inside of the _mouth_. Are there _teeth_? If so where are they situated, and how arranged? Note that all of the teeth point backwards. Food is not chewed. When some object of prey, a frog, or mouse, for example, is seized, the teeth hold it fast to the roof of the mouth and by a backward and forward movement of the lower jaws it is gradually drawn into the large œsophagus. What is the character and situation of the _tongue_? Just behind the tongue note the narrow slit, _glottis_, opening into the _windpipe_, or _trachea_. Back of the trachea opens the _œsophagus_. When the snake is laid open the elongate _heart_ will be conspicuous in the anterior third of the body. Insert a blowpipe or quill into the glottis just back of the tongue, and inflate the _lung_, which is a long, thin-walled bag extending from the region of the heart posteriorly for two-thirds of the length of the body. There is but one developed lung, the right; note at the anterior end of the lung a small mass of tissue, the atrophied left lung. Running forward from the lung is a long tube composed of incomplete cartilaginous rings, connected by membrane, the _trachea_. Note the long straight _alimentary canal_. Distinguish the _œsophagus_, _stomach_, _intestine_, _rectum_ and the _anus_. In the region of the lung is an elongated dark-red glandular mass, the _liver_. The secretion from the liver passes down through the long _hepatic duct_ to the oval-shaped green _gall-bladder_ and into the intestine. TECHNICAL NOTE.--The bile-duct may be injected through the gall-bladder with some colored injecting mass. Note that the duct running off from the gall-bladder to the intestine passes through a pink glandular organ, the _pancreas_. At the anterior end of the pancreas is a dark-red nodular structure, the _spleen_. The alimentary canal, the liver and the spleen are all suspended from the dorsal wall of the body-cavity by a delicate sheet of tissue. What is this? This condition we have also noted in the toad and fish.