CH. XXIV. 1 RESPIRATION. 337 



quadrigemina. It contains a central cavity lined with ciliated 

 epithelium. The gland substance proper is divisible into (i) 

 An outer cortical layer, analogous in structure to the anterior 

 lobe of the pituitary body; and (2) An inner central layer, 

 wholly nervous. The cortical layer consists of a number of 

 closed follicles, containing (a) cells of variable shape, rounded, 

 elongated, or stellate ; (l>) fusiform cells. There is also present 

 brain-sand, gritty matter consisting of round particles aggregated 

 into small masses. The central substance consists of white and 

 grey matter. The blood-vessels are small and form a very delicate 

 capillary plexus. 



The pineal gland is the atrophied remains of a third eye situated 

 centrally. This eye is found in a more perfect condition, though 

 covered by skin, in certain lizards, such as Hatteria. 



The Coccygeal and Carotid Glands. 



These so-called glands are situated, the one in front of the tip of 

 the coccyx and the other at the point of bifurcation of the common 

 carotid artery on each side. They are made up of a plexus of 

 small arteries, are enclosed and supported by a capsule of fibrous 

 tissue, which contains connective-tissue corpuscles. The blood- 

 vessels are surrounded by one or more layers of cells resembling 

 secreting-cells, which are said to be modified plasma cells of the 

 connective-tissue. The function of these bodies is unknown. 



CHAPTER XXIV. 



RESPIRATION. 



i 



THE respiratory apparatus consists of the lungs and of the air- 

 passages which lead to them. In murine animals the gills fulfil 

 the same functions as the lungs of air-breathing animals. The 

 muscles which move the thorax and the nerves that supply them 

 must also be included under the general heading Respiratory 

 System ; and, using this expression in the widest sense, it includes 

 practically all the tissues of the body, since they are all concerned 

 in the using up of oxygen and the production of waste materials, 

 like carbonic acid. 



Essentially a lung or gill is constructed of a thin membrane, 

 one surface of which is exposed to the air or water, as the case 

 may be, while, on the other, is a network of blood-vessels 

 K.P. z 



