68 DESCRIPTIONS OF PREPARATIONS. 



overlap one another. They are processes of the dermis with an epidermis, 

 the outer layers of which are thick and horny. These layers, together with 

 the outer coat of the fused eyelids and the labial organs of sense, are 

 moulted periodically, apparently at intervals of one month, during the 

 active summer life of the animal. There are three types of scales : the flat 

 plates of the head with apposed edges : the series of broad ambulatory 

 ventral scales extending from the throat to the divided scale which pro- 

 tects the cloaca: and the triangular scales of the body, smallest, and strongly 

 keeled near the median dorsal line. 



The Harderian gland of the eye and the labial glands of the mouth 

 have been exposed by the removal of the skin. The lobed extremity of the 

 former appears behind the eye : it extends below that organ and opens by 

 a single duct at the inner angle. The true lacrymal gland which lies above 

 the eye is absent in Serpents but present in Lizards, e. g. the Blind-worm. 

 The labial glands of the upper jaw are divisible into two kinds, readily 

 distinguishable in the freshly killed animal. The first kind is grey in colour 

 and forms (i) the azygos rostral gland lying upon the praemaxillae and not 

 exposed here, and (2) a series of composite glands, each with its own duct 

 extending back to the angle of the gape. The second kind is naturally 

 white, but assumes a yellow colour in spirit preparations. It is large, and 

 opens by a single duct between the maxillary teeth, and is the homologue 

 of the poison gland in the Viper. The glands of each ramus of the lower 

 jaw form an unbroken series and are grey in colour. 



The integument has been divided m the median ventral line as far 

 as the prae-cloacal scale, and then reflected to the right and left. The 

 dark-coloured tongue formed by the hyoglossi muscles extends backwards 

 from the chin. On either side of it is a white rod, generally considered to 

 be the larger cornu of the hyoid bone. The common jugular veins lie to 

 the outer side of these rods. They rise into view at the angle of the man- 

 dible, and the right vein is well displayed in its whole course, the left only 

 close to the heart. The trachea is seen externally to the right jugular close 

 to the heart. This portion of it is dilated and marked by irregular ridges : 

 the first portion lies dorsally to the tongue and has complete rings. The 

 oesophagus, at first dorsal to the trachea, passes to the left side of the body 

 where it becomes visible close to the heart. It dilates gradually into the 

 stomach. Between the two jugular veins, and close to the heart, is the 

 thymus gland, and between the latter and the heart three vessels are visible, 

 one in front, the left aorta, one difficult to see in the middle, the carotid, 

 and a third passing to the right, the right aorta. The left aorta reappears 

 at the outer side of the left jugular vein, embracing the oesophagus. This 

 organ is twisted to show the dorsal junction of the two aortae, beneath which 

 a black bristle has been passed. The dorsal aorta in Snakes is not closely 

 tied to the backbone as it is in other Vertebrata. 



