i 5 2 MORPHOLOGICAL TYPES 



grows down into the scrotal sac and guides the testis thither 

 in its descent. 



It may be mentioned that the ovary in mammals is peculiar 

 in possessing Graafian follicles (see p. 227). 



Ductless Glands (see Chapter XXXIII). — The spleen is 

 situated in the mesentery near the stomach. It is related to 

 the lymphatic glands, and its function is to act as a filter or 

 purifier of the blood. This it does by destroying worn-out 

 blood-corpuscles, and foreign bodies which may have got into 

 the blood. 



The thyroid is two-lobed, and lies across the ventral side 

 of the larynx. It is associated with the parathyroids. 



The thymus lies close in front of the heart, and is smaller 

 in older than in younger animals. The adrenals are small 

 compact bodies lying anterior to the kidney on each side. 

 Each consists of a cortex (corresponding to the inter-renal of 

 Scyllium) and a central medulla (supra- renal). The pituitary 

 lies in a depression in the floor of the skull, called the sella 

 turcica. The gland is composed of four parts : anterior, 

 intermedia, tuberalis, and nervosa. The pineal gland is on 

 the roof of the between-brain and between the cerebral hemi- 

 spheres. The pancreas has already been noticed on account 

 of its external secretion into the duodenum, but it also has a 

 very important internal secretion formed by the islets of 

 Langerhans. The gonads produce an internal secretion which 

 is responsible for the differentiation of the sexual characters 

 of their particular sex, but it is not yet clear which tissue is 

 responsible for this effect. In the pregnant female, the follicle 

 from which the egg was liberated becomes a corpus luteum, 

 the internal secretion of which plays an important part in the 

 development of the embryo in the uterus. 



Nervous System. — The most important characteristics of 

 the mammalian nervous system are to be found in the brain. 



The medulla oblongata or myelencephalon is not very 

 different from that of lower forms, but in the metencephalon 

 the cerebellum is much enlarged and divisible into a number of 

 lobes. Its surface is thrown into a number of folds, which 

 increases the quantity of superficial grey matter or cortex 



