xii.] THE EXCRETORY ORGANS. 485 



vertebral artery, or both. It may lie between the diverging 

 carotids, as in Python and Chelonia, or on the carotids, close 

 to the basi-branchials, as in the Frog. It is thus generally 

 connected with the primitive arteries to which the aorta 

 gives rise. 



14. The THYMUS GLAND of man is but a temporary struc- 

 ture, which disappears in the adult, and is at its maximum of 

 size at about the end of the second year. It is then long and 

 narrow, and placed partly in the chest and partly in the neck 

 (between the sternum and the great vessels), and having the 

 appearance of a ductless gland. It is irregular in shape, 

 with a considerable internal cavity, but it is more or less com- 

 pletely divided into two elongated lateral lobes, which taper 

 upwards. At birth it measures about two inches in length. 



In that man develops a thymus gland he agrees with air- 

 breathing Vertebrates generally. 



It appears to be wanting in Proteus and Siren, and to be 

 developed in the Tadpole of the Frog only with the develop- 

 ment of the lungs, disappearing again in the adult, and being 

 transformed into fat. It is often, as in the Frog, a double 

 gland one on each side. In the Chick the thymus soon 

 aborts, but is present, when the animal is a week old, as two 

 hollow tubes placed one on each side of the neck. 



The thymus may be broad and flat, covering the thyroid 

 sternally, as in Iguana. It may send up, on each side, a pro- 

 cess within the angle of the mandible, and may so form a 

 large mass beneath the skull, as in the Calf. 



15. The CUTANEOUS GLANDS of the human body are 

 insignificant in size, with the exception of that special agglo- 

 meration of them which constitutes the MAMMARY GLAND, 

 or breast. 



They are of two kinds sebaceous glands and sudoriferous 

 or sweat glands. 



(1) The sebaceous glands (noticed in Lesson V. 32 of 

 the "Elementary Physiology") are each a cluster of small 

 secreting tubes placed in the dermis, and discharging their 

 fatty secretion, by a small duct, usually into the sheath or 

 follicle of one of the hairs. 



(2) The sudoriferous glands (noticed in Lesson V. 16 

 of the " Elementary Physiology ") consist each of a fine 

 secreting tube, coiled up into a ball, placed beneath the 

 dermis, in the subcutaneous tissue, and pouring out its con- 

 tents by a delicate convoluted tube opening by a pore on the 

 surface of the epidermis. 



